Malaria Vaccine R&D in the Decade of Vaccines

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Vaccine 31S (2013) B233–B243

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Review

Malaria vaccine R&D in the Decade of Vaccines: Breakthroughs, challenges and opportunities

a,∗ b c d a

Ashley J. Birkett , Vasee S. Moorthy , Christian Loucq , Chetan E. Chitnis , David C. Kaslow

PATH Malaria Vaccine Initiative, Washington DC, USA

Initiative for Vaccine Research, Department of Immunization, Vaccines & Biologicals, World Health Organization, Geneva, Switzerland

International Vaccine Institute, Seoul, South Korea

International Centre for Genetic Engineering and Biotechnology, New Delhi, India

a r t i c l e i n f o a b s t r a c t

Article history: While recent progress has been made in reducing malaria mortality with other interventions, vaccines are

Received 13 August 2012

still urgently needed to further reduce the incidence of clinical disease, including during pregnancy, and

Received in revised form 6 February 2013

to provide “herd protection” by blocking parasite transmission. The most clinically advanced candidate,

Accepted 25 February 2013

RTS,S, is presently undergoing Phase 3 evaluation in young African children across 13 clinical sites in eight

African countries. In the 12-month period following vaccination, RTS,S conferred approximately 50% pro-

tection from clinical Plasmodium falciparum disease in children aged 5–17 months, and approximately

Keywords:

30% protection in children aged 6–12 weeks when administered in conjunction with Expanded Program

P. falciparum

for Immunization (EPI) vaccines. The development of more highly efﬁcacious vaccines to prevent clinical

P. vivax

Malaria disease caused by both P. falciparum and Plasmodium vivax, as well as vaccines to support elimination

Vaccine efforts by inducing immunity that blocks malaria parasite transmission, are priorities. Some key barri-

Elimination ers to malaria vaccine development include: a paucity of well-characterized target immunogens and an

Eradication absence of clear correlates of protection to enable vaccine development targeting all stages of the P. falcip-

malERA

arum and P. vivax lifecycles; a limited number of safe and effective delivery systems, including adjuvants,

Malaria Vaccine Technology Roadmap

that induce potent, long-lived protective immunity, be it by antibody, CD4+, and/or CD8+ T cell responses;

and, for vaccines designed to provide “herd protection” by targeting sexual stage and/or mosquito anti-

gens, the lack of a clear clinical and regulatory pathway to licensure using non-traditional endpoints.

Contents

1. Introduction ...... B233

2. Overview of malaria vaccines in development ...... B234

3. Vaccines to prevent infection and/or clinical disease ...... B235

3.1. Pre-erythrocytic vaccines ...... B236

3.2. Asexual blood-stage vaccines ...... B238

4. Vaccines to prevent malaria parasite transmission ...... B239

5. Summary ...... B241

Acknowledgements ...... B242

Conﬂict of interest ...... B242

References ...... B242

1. Introduction

Malaria is caused by ﬁve species of Plasmodium that infect

∗ humans (Plasmodium falciparum, Plasmodium vivax, Plasmodium

Corresponding author at: PATH Malaria Vaccine Initiative, 455 Massachusetts

ovale spp., Plasmodium malariae and Plasmodium knowlesi) and is

Avenue NW, Suite 1000, Washington, DC 20001-2621, USA. Tel.: +1 202 540 4432;

transmitted by the bite of infected female Anopheline mosquitoes.

fax: +1 202 457 1466.

E-mail address: [email protected] (A.J. Birkett). In 2010, approximately 3.3 billion people were at risk of malaria;

B234 A.J. Birkett et al. / Vaccine 31S (2013) B233–B243

Table 1

there were an estimated 216 million cases and an estimated

General considerations for malaria vaccine development: key challenges and

655,000 deaths [1]. The vast majority of clinical cases (81%) and

opportunities.

deaths (91%) occur in sub-Saharan Africa, with children under

Key challenges Opportunities

ﬁve years of age and primigravid pregnant women most affected

[1]. Between 2000 and 2010, the estimated incidence of malaria • Clear alignment on • Revise Malaria Vaccine Technology

declined by 17% and malaria-speciﬁc mortality rates by 26% [1]. community goals for malaria Roadmap to more effectively

vaccine development communicate community goals,

Despite these encouraging gains, associated with the scale-up of

particularly with respect to P. vivax and

preventive, diagnostic and treatment measures, new interventions,

the need for vaccines to interrupt

including vaccines to prevent clinical disease and transmission, are

malaria transmission

• •

urgently needed [2]. This paper provides a synopsis of the state Ensuring vaccines under Develop TPPs with input from key

development have proﬁle to stakeholders, including national

of current malaria vaccine development efforts, with particular

meet global need authorities in endemic countries

emphasis on the impact of the possible availability of a ﬁrst gen-

•

Pharmacovigilance studies for • Strengthening of the routine

eration malaria vaccine in the near future, and highlights some of

vaccines developed exclusively pharmacovigilance surveillance

the key product development challenges and opportunities that for use in developing world systems in developing countries and

will need to be considered to ensure maximal progress during the the establishment of a network of

pharmacovigilance sentinel sites,

current decade.

including via the Global Vaccine Safety

Since 2006, the primary strategic guidance document inform-

Initiative [23]

ing malaria vaccine development has been the Malaria Vaccine • •

Absence of dual market Wherever possible, prioritize

Technology Roadmap, which was developed through an extensive opportunity for malaria cost-efﬁcient delivery and adjuvant

vaccines to help offset the platforms for which signiﬁcant safety

consultative process [3]. In recognition of the disproportionate and

development burdens databases are available from developed

massive disease burden in young African children, the vision, as

world products

well as landmark and strategic goals, focused primarily on this

target population and geography, with an exclusive focus on P.

falciparum morbidity and mortality and little discussion of the

P. vivax, and other strategic changes and thus re-launch an up-to-

transmission perspective. To accelerate progress toward achieving

date strategic goal.

the vision and goals, a series of 11 priority areas were described that

were broadly categorized under research, vaccine development,

key capacities, and policy/commercialization [3]. 2. Overview of malaria vaccines in development

At the Malaria Forum convened by the Bill & Melinda Gates

Foundation in October 2007 and with the support of the World A comprehensive database summarizing the malaria vaccines

Health Organization (WHO), the Roll back Malaria (RBM) Partner- under development (‘Malaria Vaccine Rainbow Tables’) is main-

ship, and many other key stakeholders, there was a re-afﬁrmation tained by WHO [9,10]. Two strategic goals from malaria vaccine

that the ultimate long-term goal of the global community must be use are currently being sought: the ﬁrst targets induction of immu-

malaria eradication [4]. In 2008, the Malaria Eradication Research nity to prevent clinical disease, be it uncomplicated or severe,

Agenda (malERA) initiative was convened to deﬁne the knowl- and the second targets induction of immunity to interrupt trans-

edge base, strategies, and tools required to eradicate malaria from mission and thereby support elimination and eradication efforts

the human population [5]. The outcome of this consultation was [11]. In both cases, vaccines are needed that target all Plasmodia

a research and development agenda for regional elimination and species that cause human disease, but most notably P. falcip-

eventual global eradication of malaria, communicated in the form arum and P. vivax. Vaccines to prevent clinical disease target

of a series of manuscripts, outlining the transformational tools and pre-erythrocytic (PE) and/or asexual blood stage (BS) antigens, and

strategies, including vaccines, needed to support malaria eradica- are primarily intended for those enduring the greatest burden of

tion [2,6]. disease; whereas, vaccines targeting transmission primarily target

A series of general challenges in the quest to develop vac- PE and/or sexual, sporogonic or mosquito stage (SSM) antigens, and

cines that are effectively aligned with community goals to reduce are targeted to populations at risk of endemic transmission. While it

and eventually eliminate the suffering from malaria are outlined may be possible to develop vaccines that are highly effective at both

in Table 1. In February 2012, consensus was reached at a WHO preventing clinical disease (i.e., cases averted, toward saving lives

Scientiﬁc Forum held in Geneva to update the Malaria Vaccine and preventing disease) and interrupting the cycle of transmission

Technology Roadmap. It is expected that the update will broaden (i.e., transmission interrupted, to support control and elimination),

the Roadmap Vision and Strategic Goal(s) to include: a larger geo- they are associated with distinct clinical endpoints, overlapping but

graphic scope, older age groups, species beyond P. falciparum, and different target populations, discrete TPPs, regulatory approval pro-

the need for vaccines to reduce transmission in addition to mor- cesses, and implementation strategies. It is therefore reasonable to

bidity and mortality [7]. It will be critical that vaccines under consider both classes independently, particularly during the early

development have the proﬁle to meet global needs, with robust stages of development when conclusive clinical data to support

Target Product Proﬁles (TPPs) expected to be an essential tool in either indication is lacking (Fig. 1). Projecting forward to the ulti-

this regard [6,8]. Given the lengthy development time for vaccines, mate goal of regional elimination and eventual global eradication, it

the TPPs should be clearly deﬁned, but may need to be revisited if is anticipated that both types of vaccine will serve important roles

critical new information emerges that impacts the desirable and/or when effectively implemented in conjunction with other tools.

minimum criteria for the vaccines under development. Unlike many other vaccines, for which there is a dual market

The original 2015 Landmark identiﬁed in the 2006 Roadmap, to ﬁnancially support their introduction into developing countries,

‘to develop and license a ﬁrst-generation malaria vaccine that has most, if not all, classes of malaria vaccines, but particularly those to

a protective efﬁcacy of more than 50% against severe disease and prevent transmission, will have a single dominant market, primar-

death and lasts longer than one year’, as well as the 11 priority ily the public market in low-income countries. This is a challenge

research areas, toward which signiﬁcant progress has been made, shared with the development of other vaccines, such as typhoid

are not part of the Roadmap updating process currently under- and Japanese encephalitis. While there are additional, more ﬁnan-

way. The 2025 goal will be revised through public consultation cially lucrative markets for malaria vaccines offering protection

in order to include morbidity and transmission, P. falciparum and from infection and/or clinical disease, such as travelers, military,

A.J. Birkett et al. / Vaccine 31S (2013) B233–B243 B235

Fig. 1. Association between malaria lifecycle stage targeted, clinical outcome and anticipated goal/impact. Vaccine approaches that target only sexual, sporogonic and/or

mosquito antigens are expected to have a role in control and elimination. Vaccine approaches targeting only asexual blood-stage antigens are expected to reduce malaria

cases (i.e., cases averted) by preventing malaria disease. Targeting pre-erythrocytic-stage antigens can contribute to both outcomes (i.e., transmission interrupted and cases

averted), be preventing new infections. * In developing vaccines that function by preventing clinical cases, an important consideration is that the vaccine should not evoke

an asymptomatic carrier state that could increase transmission and clinical cases at the community level.

expatriates and middle class populations in endemic areas, these episode of clinical malaria [16]. It is understood that the total num-

vaccines would likely require very high-levels of efﬁcacy to replace ber of episodes in trial participants is a more informative endpoint

currently available chemoprophylaxis and may have different in terms of public health, but at the time of the 2006 consultation

requirements for durability of protection. This places immense there were concerns about statistical methodology for all episodes

pressure on the ﬁnancial aspects of malaria vaccine development measures. It is now recognized that appropriate statistical method-

and implementation, particularly those associated with late-stage ology exists to evaluate efﬁcacy against all episodes of malaria [16].

clinical studies, development of low-cost vaccine production, and In this context, future vaccine trials aimed at estimating vaccine

post-approval commitments (e.g., pharmacovigilance, safety, and efﬁcacy against clinical malaria should consider calculating vac-

effectiveness studies). One opportunity to help overcome this cine efﬁcacy using a comparison of the total numbers of episodes of

challenge is to prioritize cost-efﬁcient vaccine technologies and malaria, such as the incidence rate ratio. Additionally, the duration

delivery platforms for which signiﬁcant safety databases exist (see of protection against clinical manifestations will be a critical guid-

Table 1). ing factor for future development efforts and eventual introduction

of vaccines into public health programs.

3. Vaccines to prevent infection and/or clinical disease The RTS,S Phase 3 study involves two age groups; children

aged 5–17 months and 6–12 weeks at the time of enrollment. The

Malaria vaccines to prevent clinical disease will be primarily younger children are aligned with targeting the age group for co-

targeted to those enduring the greatest burden of disease; in the administration with DTP-containing vaccines, which is a strategic

case of P. falciparum this is young African children who account for consideration based on both the burden of disease in young chil-

approximately 80 percent of the total worldwide mortality from dren and implementation considerations [14]. One-year follow-up

malaria [1]. While there is an increasing awareness of the global data in 5–17 month-old children revealed a 55.8% (97.5% CI, 50.6

morbidity burden attributable to P. vivax malaria, the increased to 60.4) reduction in the incidence of the ﬁrst or only episode of

burden of disease, particularly severe disease associated with P. fal- clinical malaria and a 47.3% (95% CI, 22.4 to 64.2) reduction in the

ciparum, combined with the signiﬁcant presence of co-infections incidence of severe malaria, when compared to the control group

outside sub-Saharan Africa, is strongly suggestive that a com- [18]. In 6–12 week-old infants, for the period 14 months after the

bination vaccine will afford the most viable approach to target ﬁrst dose of vaccine, the reduction in incidence of ﬁrst or only

P. vivax clinical disease. This section outlines speciﬁc challenges, episode of clinical malaria was 30.1% (95% CI, 23.6 to 36.1) for the

and opportunities, associated with the development of vaccines intent to treat population, and 31.3% (97.5% CI, 23.6 to 38.3) for the

to prevent malaria caused by P. falciparum and P. vivax. These are per-protocol population. Vaccine efﬁcacy against severe malaria

−

summarized in Table 2. was 26.0% (95% CI, 7.4–48.6) in the intention-to treat popula-

The most clinically advanced malaria vaccine candidate tion and 36.6% (95% CI, 4.6–57.7) in the per-protocol population

in development to prevent clinical disease, RTS,S, is a pre- [19]. The efﬁcacy against clinical malaria does not differ markedly

erythrocytic, subunit vaccine based on a single parasite antigen in the RTS,S/AS01 Phase 3 trial to date when one considers ﬁrst or

(the circumsporozoite protein, or CSP), formulated which AS01 only episodes, or all episodes of malaria over a 12 month period

adjuvant, and currently undergoing Phase 3 evaluation via a col- in either age group [18,19]. Schoenfeld residuals statistical model-

laboration between GlaxoSmithKline Vaccines (GSK), the PATH ing has indicated a nonproportionality of hazard over time, which

Malaria Vaccine Initiative (MVI), and 13 clinical sites in eight could be due to waning vaccine efﬁcacy, differential acquisition of

sub-Saharan African countries [12,13]. There has been signiﬁcant natural immunity, heterogeneity of risk or, most likely, some com-

discussion on the appropriate endpoints for measuring efﬁcacy of bination of these [18,19]. Vaccine efﬁcacy over time will be reported

malaria vaccines to prevent clinical disease in pivotal efﬁcacy stud- over both a 30 month follow-up period, as well to 18 months after

ies [15,16]. The consensus of a study group convened by WHO in completion of primary course (Month 20) and from 18 months

2006, which informed the design of the RTS,S study, was that the to 30 months [17]. Currently available data suggest that the vac-

primary endpoint in a pivotal Phase 3 trial should be time to ﬁrst cine candidate is generally safe and well tolerated [18,19]. Safety

B236 A.J. Birkett et al. / Vaccine 31S (2013) B233–B243

Table 2

Malaria vaccines providing direct, immediate beneﬁt via prevention of mosquito-to-human transmission and/or clinical disease: key challenges and opportunities.

Key Challenges Opportunities

•

• Absence of biomarkers of protection for RTS,S, IMRAS, ITV, and for Identiﬁcation of surrogate markers of protection and increased understanding of

naturally acquired blood-stage immunity mechanism of protection

• Increased interrogation from human challenge and ﬁeld efﬁcacy studies including

use of systems biology approach

•

Field testing of next-generation vaccines if RTS,S/AS01 is Maximize use of surrogates of immunity in product development

recommended • Next-generation vaccines incorporating RTS,S/AS01 as one component

• Poor persistence of protection in endemic ﬁeld studies • Greater use of CHMI to study durability of protective responses, preferably in

volunteers not receiving challenge immediately following immunization

•

• Absence of reliable functional assays with established relevance to Evaluation of assays in context of search for correlates, including improved assays for

protection in humans measuring biological function, such as sporozoite migration and invasion of

hepatocytes, blood-stage growth (GIA and ADCI) sequestration of P.

falciparum-infected erythrocytes (IE) in the placenta, and greater consideration of

assays monitoring cytotoxic effects of antibodies

•

Limited availability of vaccine target antigens, and long timelines to Systematic interrogation of sporozoite-, liver- and blood-stage antigens for

valid targets in humans suitability as vaccine antigens

• Prioritization of targets associated with essential biological function in parasite and

broad strain coverage

• Development of panels of monoclonal antibodies, preferably with functional activity,

to support antigenicity assessment, prior to advancing to time-consuming preclinical

immunogenicity studies

• •

Limited availability of effective delivery systems to induce strong and Employ clearly deﬁned preclinical go/no-go criteria and maximize use of CHMI

durable antibody, Th1 CD4+ and CD8+ T-cell responses to multiple model to test most promising platforms

antigens • Leverage delivery platform and adjuvant data from other disease areas

• Strong reliance on prime-boost approaches • Assess and maximize feasibility of prime-boost implementation in ﬁeld

•

Absence of reproducible challenge model for P. vivax that can be Establishment of P. vivax continuous culture to establish well-characterized

broadly implemented challenge strains, as well as availability of safe and effective drug treatments to clear

hypnozoites, toward establishment of human challenge model

and immunogenicity studies in HIV-infected children are ongoing all safety data, pre- and post-approval, must be gathered in devel-

[20,21]. oping countries. For RTS,S, the value of pharmacovigilance studies

A policy recommendation for RTS,S could be made as early as has been heightened by the use of a novel adjuvant system, AS01,

2015, depending on ﬁnal results from Phase 3 trials expected in which has not been commercialized in other products and there-

late 2014 [3]. To assess the public health impact of malaria vac- fore is not associated with supplemental safety databases on which

cines targeting clinical disease, vaccine efﬁcacy will need to be to draw [22]. Since future malaria vaccines will also see primary use

considered in the context of the ongoing burden of disease (attack in low-income countries, the need for safety data is an important

rate), in the target populations, to estimate the projected number consideration, particularly with regard to the enhanced pharma-

of cases averted. The apparent differences in estimated efﬁcacy covigilance burden that novel vaccine platforms will confer. This

results and in immunogenicity data between the two age cate- need highlights the value of, where possible, developing malaria

gories in the RTS,S/AS01 Phase 3 study populations highlight gaps in vaccines using cost-effective vaccine platform technologies, such as

our understanding of the factors associated with protective efﬁcacy delivery systems and adjuvants, for which substantial human safety

and with the induction of immune responses, and how they con- databases are already available. The development of novel vaccines

fer protective efﬁcacy, in infants and young children. The authors intended speciﬁcally for developing country populations also high-

of the manuscript reporting these data have hypothesized that lights the need to strengthen vaccine safety and pharmacovigilance

one or more of a number of different factors may be associated systems in developing countries, including via the Global Vaccine

with the ﬁndings, including, immune interference associated with Safety Initiative (Vaccine Safety Blueprint), [23].

co-administration of EPI vaccines, maternal immunity, immune

system development status, and different transmission intensities 3.1. Pre-erythrocytic vaccines

across the study sites [14,15]. Additional data are needed to provide

clarity in this regard, both to inform the continued development of The absence of a biomarker of protection for human immu-

RTS,S as well as other vaccine approaches targeting young infants. nity approaches, such as Immunization via mosquito bite with

Vaccines introduced in developing countries are typically asso- radiation-attenuated sporozoites (IMRAS), infection-treatment-

ciated with years, if not decades, of safety data gathered in the vaccination (ITV), and RTS,S, that have been associated with

resource-rich countries in which they were ﬁrst introduced [8]. signiﬁcant levels of protection in vaccinated volunteers in con-

While this delay to implementation has led to signiﬁcant excess trolled sporozoite challenge studies in humans, is a signiﬁcant

morbidity and mortality in developing countries, it has gener- knowledge gap that, if ﬁlled, would almost certainly acceler-

ally alleviated the burden on the health systems of developing ate development of next-generation vaccines [24–26]. Efforts are

countries to perform complex pharmacovigilance studies. These ongoing to identify biomarkers via controlled human malaria infec-

studies require extended follow-up of large numbers of vaccine tion studies, and via ﬁeld efﬁcacy trials in young African children.

recipients, to enable detection of rare adverse events, and are cap- The distinct clinical endpoints for such studies, typically infection

tured via both active case detection and establishment of systems in human challenge studies versus clinical disease in the ﬁeld, as

for passive collection of safety data. These data are extremely chal- well other limitations (such as the limited volume of blood sam-

lenging to capture, particularly in environments where healthcare ples taken from small children), represent signiﬁcant challenges.

systems are less well developed and baseline data on possible In addition the fact that RTS,S appears to confer partial protection

adverse events of speciﬁc interest after immunization are limited. to a group of vaccinees, rather than complete protection to a pro-

The development of RTS,S, which is solely intended for introduction portion of vaccinees has implications for study design in the search

in malaria-endemic countries, highlights this challenge: essentially for correlates. For example traditional case control studies and the

A.J. Birkett et al. / Vaccine 31S (2013) B233–B243 B237

concept of “breakthrough” cases need to be revised with this type conferred by whole-parasite immunogens such as IMRAS and ITV.

of vaccine protection. Miniaturization of assays would improve In both scenarios, functional assays and challenge models will be

our capacity to analyze small blood volumes from pediatric tri- critical to support progression to clinical evaluation. Humanized

als performed in the ﬁeld. Interrogation of immune responses and mice that support infection by human Plasmodium sp. may be of

gene expression patterns (innate immune signature), from pro- particular value in preclinical target validation, but will likely need

tected and non-protected vaccinated individuals participating in to be preceded by more high throughput assays and used primarily

human challenge studies, offers an ideal environment for identi- as a ‘ﬁnal conﬁrmatory screen’. It will be important to ensure that

fying correlates of protection against infection. RTS,S/AS01 confers capacity for future challenge studies is aligned with the emerging

approximately 50% (95% conﬁdence interval [CI], 32.9–67.1%) pro- vaccine approaches and experimental medicine studies.

tection from infection in the human challenge model and therefore A CHMI challenge model for P. vivax has been implemented at

provides an ideal scenario for identifying correlates [25]. In addition two clinical centers [36–38]. P. vivax CHMI relies on acquisition of P.

to humoral and cellular biomarkers, the use of a systems biology vivax-infected mosquitoes by feeding them on blood from infected

approach, applied to human challenge studies, may offer a more donors, or, potentially, the banking of P. vivax-infected red blood

comprehensive strategy to identify biomarkers that could subse- cells, for a blood-stage challenge. P. vivax sporozoite challenge stud-

quently be conﬁrmed in ﬁeld studies. ies are further complicated by the possibility of relapse in a small

Malaria vaccine developers are indeed fortunate to have access proportion of study volunteers. The development of a robust con-

to a well-established controlled human malaria infection (CHMI) tinuous culture system for P. vivax would be signiﬁcantly enabling

model for P. falciparum in which infectious sporozoites are admin- to the malaria vaccine development community, including for the

istered to immunized volunteers in early clinical trials via the bite establishment of a robust P. vivax CHMI model.

of infected mosquitoes [27]. This model has proven critical in con- A promising approach for developing vaccines with improved

ﬁrming the biological feasibility that high levels of protection from efﬁcacy is heterologous prime-boost, in which a common antigen,

infection can be achieved in human volunteers via IMRAS (requiring or combination of antigens, is delivered using two different deliv-

approximately 1000 immunizing mosquito bites) and ITV (requir- ery systems that are administered in a distinct order. This approach

ing just 45 immunizing mosquito bites) [24,26,28,29]. The former is viewed as particularly valuable in the development of pre-

is being pursued as a vaccine approach, although the high levels erythrocytic vaccines, to maximize humoral and cellular immunity

of protection from infection observed following immunization via to block initial hepatocyte invasion and effectively target liver-

1000 mosquito bites have not been observed when similar num- stage parasites. Despite the paucity of promising pre-erythrocytic

bers of sporozoites are delivered via parenteral routes [30]. ITV vaccine target antigens, and many failed attempts over the past

is of particular interest not only due to the relatively low immu- 10–15 years, two independent prime-boost approaches recently

nizing dose, but due to recent evidence of robust protection from demonstrated modest levels of protective efﬁcacy in human chal-

infection that persists for over 2 years. Evidence for heterologous lenge studies using distinct vector (pDNA, MVA, ChAd63, and Ad5)

protection for ITV has not yet been shown, while preliminary sup- and antigen combinations (ME-TRAP, AMA1, and CSP). Prelimi-

portive data for IMRAS has been reported [28]. CHMI models have nary data suggest that high levels of CD8+ T cells are associated

not been extensively used to study persistence of protection. Gen- with protection using these vaccine approaches, but larger stud-

erally, small numbers of individuals protected on initial challenge ies are needed to conﬁrm these ﬁndings. Platform optimization to

administered shortly after completion of the immunizing regimen, overcome any obstacles to anti-vector immunity, and safety. As a

have been rechallenged at later time points [25,28,31–33]. In the general point for translating challenge study results to the ﬁeld,

future, more extensive use of CHMI to interrogate the persistence the selection of optimal antigenic constructs will need to consider

of protection, preferably involving groups of volunteers receiving strain variability in the ﬁeld and human genetic variability of tar-

only a delayed challenge, may help to better predict the durability get populations, which can only be effectively evaluated in ﬁeld

of protection prior to advancing to ﬁeld studies. efﬁcacy trials.

The CHMI model has been instrumental in accelerating the Prime-boost approaches that incorporate RTS,S offer one possi-

development of RTS,S, as well as supporting go/no-go decisions for ble strategy for development of next-generation vaccines, as they

other vaccine approaches prior to advancing them to endemic ﬁeld leverage investments in clinical studies yielding an expanding

studies [34]. Furthermore, since Phase 3 studies for next-generation safety and efﬁcacy database, as well as in commercial manufac-

vaccines are expected to be initiated after 2015, demonstration of ture preparedness. An attractive opportunity for building on RTS,S,

superiority in ﬁeld studies over RTS,S (if market-approved and rec- using prime-boost strategies, is to evaluate the protective efﬁcacy

ommended for use) may be required, strengthening further the of RTS,S, conferred with no contribution from CD8+ T cells, in the

critical role of the CHMI model. To date, the model has been used context of a strong CD8+ T-cell response [28,29]. Preclinical data

primarily in the United States and Europe, but several groups in strongly support the ability of CSP to confer protection via CD8+ T

endemic regions are now exploring the possibility to establish cells, but in humans this remains to be demonstrated [39].

it to evaluate the value of performing studies in exposed indi- Despite their potential for meeting protective efﬁcacy targets,

viduals. Further, efforts are underway to develop cryopreserved heterologous prime-boost vaccine approaches are associated with

sporozoites that could be delivered by needle and syringe to over- some hurdles [40,41]. Among other factors, this includes the poten-

come the insectary requirements [35]. In addition, the CHMI model tial regulatory/implementation challenges of administering two

may have the potential to serve as a tool for use in experimental vaccine components in a speciﬁc order in the ﬁeld. A coordinated

medicine studies to answer speciﬁc immunology/efﬁcacy ques- effort involving the HIV/AIDS, Tuberculosis, and malaria vaccine

tions that could inform fundamental vaccine design. In addition to development communities appears most appropriate, given their

the identiﬁcation of biomarkers of protection for highly efﬁcacious common interest in using this novel vaccination approach to

whole-parasite vaccines, there remains an opportunity for efﬁcient develop highly effective vaccines for the developing world [42].

target validation, such as via the use of passively transferred anti- To date, only a single target antigen, PfCSP, has proven to

bodies or T cells, with defined specificities and functions, prior to afford significant efficacy in both CHMI studies and field studies.

initiating expensive and lengthy vaccine development programs. PvCSP/AS01 has been evaluated in a clinical study, including with

Two distinct approaches to enable selection of novel target antigens sporozoite challenge, but robust protection has not been attained

are: (1) Hypothesis-based research to identify speciﬁc targets based [38]. This may be related to the absence of the hepatitis B surface

on biological function; and (2) Broad interrogation of immunity antigen virus-like particle (VLP) delivery system, compared with

B238 A.J. Birkett et al. / Vaccine 31S (2013) B233–B243

RTS,S, and highlights the importance of particle-based delivery are typically manufactured and puriﬁed in real time to support

systems in development of future malaria vaccines. This obser- assays, as well as poor infectivity levels that have been marginally

vation is consistent with many other failures to develop effective improved in recent years via the use of new hepatocyte cell lines

CS-based vaccines, and likely linked to the high anti-CSP antibody and primary hepatocytes [49]. For P. vivax, the absence of con-

titers required to confer protection [20,25]. One of the few recent tinuous culture again places signiﬁcant restriction on the ability

successes for CSP delivered using alternate platforms involved CSP to develop robust functional assays. Opportunities reside in the

(and AMA1) peptides, formulated with an inﬂuenza virosome deliv- development of assays that integrate the capacity to measure

ery vehicle (PEV3B), in which the incidence rate of clinical malaria effector mechanisms associated with antibody Fc regions, as has

episodes was half the rate of the control children between study been shown for the in vivo transgenic sporozoite neutralization

days 30 and 365 (0.0035 episodes per day at risk for PEV3B vs. assay and antibody dependent cellular inhibition (ADCI), in addi-

0.0069 for Inﬂexal(R)V; RR = 0.50 [95%-CI: 0.29–0.88], p = 0.02) [43]. tion to commonly used methods assessing only the direct blocking

This ﬁnding requires conﬁrmation in further studies. effects [50,51]. In this regard, tools such as transgenic parasites and

Despite the availability of the P. falciparum and P. vivax genomes humanized mice, which enable in vivo preclinical studies focused

since 2002 and 2008, respectively, this knowledge has not been on human Plasmodium sp., are of interest [52]. Regarding cellular

effectively used to identify promising new vaccine targets and tran- immunoassays, the malaria vaccine development community has

siting them into vaccine development [44,45]. Selection of vaccine been at the forefront in the development of novel research assays

target antigens generally focuses on either an attempt to recapitu- that are critical to identify correlates of protection, but it has per-

late naturally acquired clinical immunity (semi-empiric), whereas haps lagged behind the HIV and TB ﬁelds in the standardization

others (e.g., CSP) attempt to elicit an immune response typically and validation of T cell assays that are critical to enable reliable

not observed after natural infection(s). The distinction may have comparisons of different vaccine technologies. Signiﬁcant oppor-

the important consequence of anticipated anamnestic immune tunities exist to leverage prior qualiﬁcation and validation efforts

responses, or not, and on the rapidity of selection of vaccine escape in other ﬁelds to improve assay robustness to enable better com-

mutants. In this regard, it will be important to consider the antici- parability of data between centers and support go/no-go decisions.

pated epidemiology of malaria when next-generation vaccines are For example, the HIV Vaccine Trials Network (HVTN) laboratory has

implemented in selection of future target antigens. developed and performs validated immunologic assays, including

New antigens, which are targets of humoral and/or cellu- cellular immunoassays, to support Phase 1 through Phase 3 trials

lar immunity, are needed to ensure maintenance of a healthy and potentially for regulatory submissions [53]. This is of particular

next-generation vaccine development pipeline employing subunit importance given the signiﬁcant current focus on development of

vaccine approaches. Beyond CSP, there has been little success in next-generation vaccines that induce cellular immune responses

identifying antigens that can induce antibodies that effectively toward antigens expressed at the pre-erythrocytic stage. To ensure

block hepatocyte invasion. A more rigorous effort to identify such optimal use of key assays, qualiﬁcation and harmonization efforts

antigens, and thereby target essential biological functions that of the most promising assays, as well as establishment of central

cannot be readily overcome by immune evasion, would yield high- service and/or reference centers, are critical to ensure data are suf-

priority targets for translation into clinical candidate vaccines. The ﬁciently robust to support future product development decisions.

identiﬁcation and development of vaccine targets could be accel-

erated by the availability of large panels of monoclonal antibodies, 3.2. Asexual blood-stage vaccines

preferably with functional activity, and preferably raised against

native parasite-derived antigens. Further, the use of transgenic While immunity to asexual blood-stage antigens is an impor-

parasites, where antigens of interest from all parasite stages that tant mechanism of natural immunity to malaria in endemic regions,

are of interest for human vaccine development are expressed on deﬁned biomarkers of protection, as for pre-erythrocytic targets,

rodent or non-human primate parasite backbones, is expected to remain elusive. Identiﬁcation of such biomarkers would be highly

accelerate preclinical development [46]. enabling for subunit-based vaccine development efforts. In recent

Efforts to identify targets of cellular immunity, such as via char- years, preliminary evidence for vaccine-induced clinical efﬁcacy in

acterization of the sporozoite and liver-stage proteome, has led the ﬁeld has been generated using two leading blood-stage targets,

to mining for novel targets of cell-meditated immunity to target P. falciparum AMA1 and MSP3 [54,55]. However, as with similar

infected hepatocytes in the context of sporozoite immunization preliminary ﬁndings for other antigens, such as MSP2, further stud-

[47]. However, more powerful tools are needed to identify those ies are needed to conﬁrm these initial ﬁndings [56]. A GLURP-MSP3

antigens that are not only immunogenic, but have a causal role in fusion protein (GMZ2), formulated with aluminum hydroxide adju-

inducing protective efﬁcacy. vant, is currently undergoing a Phase 2 evaluation in approximately

Whole-parasite vaccine approaches, most notably radiation- or 2000 children, aged 12–60 months, at ﬁeld sites in four African

genetically-attenuated sporozoites, as well as whole blood-stage countries (Burkina Faso, Gabon, Ghana, and Uganda). Initial efﬁcacy

vaccines, are under development [30]. While these approaches data from this study is expected to be reported in 2013.

offer the conceptual advantages associated with whole organism- Unlike pre-erythrocytic vaccines, the development of vaccines

based immunogens, their potential remains to be conﬁrmed in targeting malaria blood stages is complicated by the ongoing uncer-

humans using traditional vaccine delivery modalities and addi- tainty as to whether the infection endpoint assessable in CHMI

tional product development challenges will need to be overcome studies should be used as a stage-gate for vaccines targeting asex-

[48]. In addition to their potential role in product development, ual blood stages. It is desirable to continue to generate comparisons

whole attenuated-parasite vaccines, as well as ITV approaches of CHMI and ﬁeld efﬁcacy data for asexual stage vaccines going

using viable parasites administered under prophylactic drug cover forward, to reduce the unknowns in this area. As an alterna-

that have yielded robust protection in humans, could prove to be tive to sporozoite challenge, a blood-stage challenge model, in

valuable resources, if effectively applied to experimental medicine which a small number of infected erythrocytes are administered

approaches directed toward identifying biomarkers of protection intravenously, and asexual-stage parasite replication rates in non-

that could inform development of subunit vaccines. vaccinated and vaccinated volunteers determined using PCR are

Robust functional assays that mimic the biological events that compared, has not yet been fully interrogated to determine if this

are targets of vaccine-induced immune intervention are needed. model, in which a non-physiologically relevant dose and adminis-

Current assays are restricted by their reliance on sporozoites, which tration route of asexual blood stage parasites can reliably predict

A.J. Birkett et al. / Vaccine 31S (2013) B233–B243 B239

vaccine clinical efﬁcacy in the ﬁeld [57]. Toward determination achievement of higher invasion blocking activity against multiple

of the utility of this model in product development, blood-stage P. vivax strains.

vaccines that have demonstrated preliminary evidence of clini- Pregnant women are particularly vulnerable to malaria dis-

cal efﬁcacy in endemic ﬁeld studies could be evaluated for their ease caused by sequestration of P. falciparum-infected erythrocytes

impact on replication rates of homologous challenge strains. In in the placenta, mediated through the parasite ligand VAR2CSA

addition to conﬁrming observations in the ﬁeld, such studies may (a P. falciparum erythrocyte membrane protein 1 family mem-

facilitate identiﬁcation of immune correlates and determination of ber) [61]. The identiﬁcation of a correlate of protection for

the predictive capacity of immune-assays, such as growth inhi- pregnancy-associated malaria, in VAR2CSA, has accelerated vac-

bition assay (GIA) and ADCI, as a functional assay for selection cine development in this area [47]. The high level of antigenic

of promising blood stage vaccine candidates. The availability of a variability in VAR2CSA has hampered vaccine development efforts

challenge model that predicts ﬁeld efﬁcacy of vaccines targeting [62]; however, recent advances in identifying conserved targets

asexual blood stages could signiﬁcantly accelerate malaria vaccine may provide new vaccine opportunities [63]. While signiﬁcant

development efforts. challenges remain to effectively deﬁne an appropriate clinical

Antigenic variability continues to be a major barrier in translat- development strategy for a vaccine target exclusively for imple-

ing these promising research ﬁndings into a product development mentation in women of child-bearing age, there are precedents

approach and will need to be overcome. The bioengineering of (human papillomavirus [HPV], tetanus toxoid and inﬂuenza) for

consensus immunogens and identiﬁcation of combinations of use of licensed vaccines indicated in this population. The availabil-

reasonable numbers of antigenically diverse antigens are two ity of vaccines that interrupts malaria parasite transmission would

approaches currently being pursued to overcome extreme anti- make vaccines speciﬁcally targeting pregnancy-associated malaria

genic variability. An alternative approach is to focus on functional, redundant.

conserved regions of blood stage antigens, for example, receptor-

binding domains of parasite ligands that mediate invasion. The

ability of such antigens to elicit strain transcending functional 4. Vaccines to prevent malaria parasite transmission

antibodies that inhibit parasite growth can be used as a selection

criterion for blood stage vaccine candidates. Targeting multiple key The need for malaria vaccines that can interrupt malaria parasite

receptor–ligand interactions involved in invasion may synergisti- transmission, and thereby support future elimination and eradica-

cally provide high levels of growth inhibition; however, it remains tion efforts, was recently highlighted by the Malaria Eradication

to be seen if these approaches can overcome the signiﬁcant plastic- Research Agenda (malERA) [6]. This has resulted in a renewed

ity that has historically been associated with leading blood-stage interest by some stakeholders in a class of vaccines that previ-

vaccine target antigens. ously received minimal ﬁnancial investment and was maintained

As with pre-erythrocytic vaccine development efforts, new tar- via a limited research endeavor with minimal product development

get antigens of blood stage immunity are needed. In this regard, activity. This section outlines some of the key challenges and oppor-

emerging insights into the biology of merozoite invasion mech- tunities speciﬁc to the development of vaccines designed to block

anisms for P. falciparum are unraveling a complex network of transmission, which are summarized in Table 3.

redundant invasion pathways that may offer promising new tar- Vaccines interrupting malaria parasite transmission (or VIMT),

gets for future blood-stage vaccine development [58]. While several effectively implemented in concert with other interventions, are

promising targets have been identiﬁed, RH5 has been associated intended to eliminate the parasite reservoir in a region targeted

with promising preclinical results demonstrating broad growth for malaria elimination. malERA described a broader concept of

inhibition activity against a diverse panel of parasite strains [58,59]. VIMTs that may consist of different components including ‘classical

Identiﬁcation of a minimal number of ligands to induce antibod- transmission-blocking vaccines’ (TBVs) that target SSM stages and

ies exhibiting maximal invasion-blocking activity against diverse aim to inhibit parasite development in the vector, highly efﬁcacious

ﬁeld strains will be critical to support optimal product develop- pre-erythrocytic vaccines that reduce asexual and sexual-stage

ment efforts. Development of multi-antigen, multi-stage vaccines parasite densities, and highly efﬁcacious blood-stage vaccines that

remains a priority, for example combining optimal blood-stage efﬁciently inhibit erythrocyte invasion and growth to reduce blood-

antigens with pre-erythrocytic approaches. For P. vivax, the chal- stage parasite densities, thereby potentially reducing malaria

lenge of effectively targeting merozoite invasion ligands appears to transmission through an effect on prevalence of sexual stages [6].

be less complex, with the Duffy binding protein (DBP) representing VIMTs, which include pre-erythrocytic and/or blood-stage anti-

the overwhelmingly dominant invasion ligand for ensuring retic- gens (i.e., PE/BS-VIMT) may, in addition, protect individuals against

ulocyte invasion; however, more work in this area is warranted malaria in case of reintroduction of malaria after elimination.

to ensure that key assumptions regarding the limited impor- The application of malaria vaccines to support regional parasite

tance of non-Duffy invasion pathways in the ﬁeld are valid, as elimination is expected to be most effective if implemented fol-

well as conﬁrmation that optimal DBP constructs are being sup- lowing successful reduction of transmission to low levels by other

ported. interventions [6]. Current maps of transmission intensity, for both

Highly reproducible immunoassays are critical to support P. falciparum (Fig. 2) and P. vivax, show that the majority of the

vaccine development activities. In recent years, the malaria com- endemic world already falls into the low transmission category that

munity has invested in the standardization of key blood-stage is effectively aligned with the optimal environment of the impact of

functional assays, such as the GIA and ADCI, to support more robust VIMT [6,64,65]. Further, the capacity of other malaria interventions

preclinical and clinical decision-making processes. Unlike the GIA, to convert high-intensity transmission settings, to low-to-

where a good level of harmonization and general conﬁdence in moderate intensity transmission settings, has been demonstrated

assays readouts has been demonstrated, the ADCI assay has proven in several endemic regions [66,67]. In the case of P. vivax, persistent

more challenging to establish. Critically, the relationship between relapse infections associated with hypnozoites afford an additional

GIA and ADCI data and clinical efﬁcacy remains to be clearly deﬁned challenge that will need to be overcome when considering effec-

and needs further study to more adequately support such heavy tive usage of VIMT [6]. The goal for regional elimination would be

reliance on these assays in go/no-go decision-making [60]. In addi- to reduce the effective reproductive rate (R0) of the parasite to less

tion, combination of DBP with P. vivax homologues of P. falciparum than 1; the consequence being that each infected human would

antigens shown to elicit parasite growth inhibition may enable give rise to less than one subsequently infected human, resulting

B240 A.J. Birkett et al. / Vaccine 31S (2013) B233–B243

Table 3

Malaria vaccines providing delayed beneﬁt by preventing human-to-mosquito transmission: key challenges and opportunities.

Key Challenges Opportunities

•

• Effective demonstration of vaccine efﬁcacy (i.e., interruption of Fully explore potential for demonstration of efﬁcacy via analytical endpoint (i.e.,

transmission) ‘mosquito endpoint’)

• Exploit earlier gametocytogenesis of P.vivax to accelerate evaluation of VIMT, using most

relevant transmission measures (DMFA and DFA), via CHMI studies

• Qualiﬁcation of key functional assays (SMFA, DMFA, and DFA) toward identiﬁcation of

correlates of transmission-blocking immunity

• Effective transmission measures in place, particularly at low transmission levels

•

Absence of clearly deﬁned regulatory approval pathway for Work closely with regulatory authorities, including in endemic counties, and with WHO,

SSM-VIMT to deﬁne licensure pathway

•

Fully explore DFA, to demonstrate effective blocking of human-to-mosquito

transmission at level of individual, as licensure endpoint

• Standardization of key functional assays (SMFA, DMFA, and DFA) toward identiﬁcation of

correlates of transmission-blocking immunity

•

• Licensure of vaccine conferring delayed clinical beneﬁt Minimize burden to demonstrate vaccine safety via use of technology platforms with

signiﬁcant clinical safety databases, including in special populations

• Explore concomitant administration of relevant agents (malaria and non-malaria)

conferring immediate beneﬁt to vaccine recipient

•

Policy and implementation Appropriate engagement of key stakeholders in endemic countries

• Product proﬁle effectively aligned with implementation via elimination campaigns

• Explore feasibility of targeting intervention to populations responsible for infecting

mosquitoes

• Effective transmission measures in place, particularly at low transmission levels

• •

Evidence of a supportive environment at the global, national, and Appropriate engagement of key stakeholders in malaria endemic countries

• community levels Effective engagement of ethics and communications experts

•

• Ensure effective commercialization strategy for VIMTs –adeveloping Fully explore opportunities with developing world partners to maximize

world, public health intervention endemic-country ownership and minimize cost

•

Development of optimal immunogens Evaluation of diverse expression systems toward production of immunogens with

conformations that most accurately mimics native antigens

•

Identiﬁcation of novel targets for induction of transmission-blocking immunity

•

• Tools for rapid assessment of vaccine candidates Development of panels of monoclonal antibodies, preferably with functional activity, to

support antigenicity assessment, prior to advancing to time-consuming preclinical

immunogenicity studies

in local elimination. In terms of the Ross MacDonald formulae, the control efforts, may successfully drive down transmission rates

classic formulae in malaria epidemiology that relate entomological to reduce the effective reproduction rate (Reffective) to below 1.0

parameters to malaria parasite transmission, vaccines are projected [6]. Accurate modeling data are critical to establish early devel-

to function by reducing ‘c’, the % of bites on infected humans that opment criteria to support go/no-go decisions and current models

infect a mosquito [68]. Implementation of VIMT, together with may not be sufﬁciently predictive to reliably support development

Fig. 2. The special distribution of P. falciparum entomological inoculation rate in 2010. The majority of areas endemic for P. falciparum malaria exhibit low transmission

≤

(EIR 5), which is aligned with the projected optimal setting for implementation of VIMTs [6].

A.J. Birkett et al. / Vaccine 31S (2013) B233–B243 B241

decisions. Further, reﬁnement of these complex mathematical transmission-blocking activity have not been reported [69–71].

models, most notably using biological and epidemiological data, Additional target antigens, including Pfs48/45, Pfs230 and AnAPN1,

is needed to ensure that they are able to reliably inform the likeli- have been associated with promising preclinical data and clinical

hood of early development data to translate into eventual impact data for these antigens will be essential. Sexual-stage antigens have

in the ﬁeld. Provision of sensitivity analyses can be helpful. typically been challenging to manufacture in recombinant systems

Vaccines designed to interrupt the cycle of transmission are due to their complex secondary and tertiary structure. It will be

anticipated to be most effective when targeting the points of tran- important to ensure, during the early stages of development, that

sition by the parasite from either mosquito to human or human optimal immunogens, presumably those that most accurately repli-

to mosquito. At these two points in its lifecycle, the malaria par- cate the structure of the native antigens, are developed. Further,

asite is present in very low numbers and may be most vulnerable the intrinsic immunogenicity of several SSM constructs has been

to immune intervention. A distinct advantage of pre-erythrocytic shown to be low in preclinical and clinical studies, rendering the

(PE) and blood-stage (BS) VIMTs are that they could offer a direct, use of potent adjuvants or carriers necessary to enhance immuno-

immediate beneﬁt to the vaccine recipient by conferring direct pro- genicity [69]. Current evidence suggests that VIMTs will require

tective efﬁcacy, as well as a delayed beneﬁt associated with local high levels of vaccine coverage, or effective targeting to human

malaria elimination. A VIMT targeting sexual, sporogonic, and/or transmitters, and implementation in relatively low EIR settings, for

mosquito antigens (SSM-VIMT) would offer a delayed beneﬁt to maximal impact [72]. The likely need for broad implementation

the vaccine recipient conferred via reduced malaria transmission. cautions the use of novel technology platforms, such as protein car-

However, opportunities exist to combine SSM-VIMTs with vaccines riers, adjuvants and vectors, in view of the likely need for increased

or drugs that provide direct and immediate beneﬁt, either against assessment of safety, compared to platforms that are in commercial

malaria or another relevant infectious disease in the target popula- products and that are already associated with signiﬁcant positive

tions. Examples would include co-administration of an anti-malaria clinical safety data. Therefore, carrier systems such as tetanus tox-

drug that provides direct-immediate beneﬁt or a vaccine for which oid, CRM197, and OMPC, and aluminum-based adjuvants are likely

there is a direct and immediate beneﬁt against another infectious to be better suited to accelerate development, if they can induce

disease of relevance in that target population (e.g., meningococcal sufﬁcient biological activity.

disease, tetanus, or HPV), if feasible. The beneﬁts and drawbacks To optimize the development process for SSM constructs, it will

of such an approach, including the major development challenges, be important that development of critical surrogate assays of efﬁ-

will need to be carefully considered before signiﬁcant resources cacy keep pace with product development activities. Membrane-

are expended on such approaches. In all cases, the use of tech- and direct-feeding assays are expected to be pivotal in supporting

nology platforms that are amenable to widespread deployment in clinical go/no-go decisions and potentially in supporting regula-

elimination campaigns and have the potential to be affordable and tory ﬁlings; however, these are complex assays that are difﬁcult

cost-effective are most desirable. to qualify due to the inherent variability associated with biological

Significant challenges remain in defining the development path- systems. Assay harmonization and qualification activities for the

way for vaccine approaches where the endpoints are reduced various feeding assays will be critical, while development of less

transmission at the level of a community. Timelines and devel- complex surrogate assays, if achievable, is likely to have a signiﬁ-

opment costs to reach licensure may be signiﬁcantly reduced, cant positive impact on development timelines.

however, if an analytical endpoint (i.e., prevention of infection of Looking forward, it will be important to channel the limited

mosquitoes in laboratory-based assays) is able to support product available resources for vaccine development toward programs that

licensure, with true efﬁcacy and/or effectiveness data collected dur- are well-aligned with broadly accepted TPPs. Early and continuous

ing post-approval studies. This could be achieved by demonstrating dialog with regulatory authorities will be necessary to ensure def-

that antibodies induced by a VIMT can block human-to-mosquito inition of a robust and practical licensure pathway. Further, early

transmission at the level of individuals enrolled in carefully mon- engagement of key stakeholders at the national, regional, and local

itored proof-of-concept studies, with determination of surrogates levels in malaria-endemic countries will be necessary to ensure TPP

of efﬁcacy to bridge to other target populations. Such studies could alignment, and fostering of optimal understanding of the possible

involve the ‘challenge’ of mosquitoes via direct-feed experiments role of SSM-VIMT for eventual implementation as a supplemental

(DFA) involving immunized, gametocyte-positive individuals or, intervention to support elimination and eradication efforts.

less preferably, using the standard membrane-feeding assay

(SMFA) and/or direct membrane-feeding assay (DMFA). The ability

to perform DFA as part of CHMI studies, prior to drug cure of study 5. Summary

volunteers, could signiﬁcantly accelerate development timelines

and reduce risk associated with the uncertain predictive capacity There is a need for new tools to support malaria control and

of SMFA and DMFA for early development decisions. The earlier elimination efforts, and different combinations of tools are likely

gametocytogenesis associated with P. vivax infection, compared to be appropriate in different settings. The malaria vaccine devel-

to P. falciparum, may present a valuable opportunity for P. vivax opment community has recently been challenged with revising

sporozoite and/or blood stage challenge models to accelerate the its vision and strategic goals for development of products that

development of VIMTs. can most effectively support these efforts. A revised vision and

Extensive dialog with regulatory authorities and national stake- strategic goal(s) are expected to be completed in 2013 through

holders in endemic countries will be needed over the coming years a WHO-coordinated consultative process. These revisions may

to establish an appropriate and practical licensure pathway. At a afford new opportunities for malaria vaccine development, most

TBV workshop organized by MVI in 2010 (unpublished), a repre- notably those associated with approaches that have not been

sentative of the FDA communicated that there was no legal bar in fully exploited previously (such as the development of vaccines

the USA to approve a vaccine that does not offer a direct, immediate to reduce transmission). This paper outlines some important

beneﬁt to vaccine recipients. Subsequently, detailed TPPs have been considerations for development of vaccines that can have a max-

developed to outline the critical characteristics of such a vaccine. imal impact in supporting the mission to more effectively control

To date, only two SSM antigens have been evaluated in Phase malaria, as well as enable regional elimination and eventually

1 studies (Pfs25 and Pvs25), delivered as either recombinant pro- global eradication, in conjunction with other malaria preventive,

teins or via attenuated vaccinia virus; however, high levels of diagnostic and treatment measures.

B242 A.J. Birkett et al. / Vaccine 31S (2013) B233–B243

Challenges remain at all stages of the vaccine development path- [15] Moorthy V, Reed Z, Smith PG. Measurement of malaria vaccine efﬁcacy in phase

III trials: report of a WHO consultation. Vaccine 2007;25(28):5115–23.

way, from discovery through implementation. Global development

[16] Moorthy VS, Reed Z, Smith PG. MALVAC 2008: measures of efﬁcacy of malaria

challenges such as the absence of a dual market opportunity for

vaccines in phase 2b and phase 3 trials – scientiﬁc, regulatory and public health

malaria vaccines to help offset the development burdens, including perspectives. Vaccine 2009;27(5):624–8.

[17] Lievens M, et al. Statistical methodology for the evaluation of vaccine efﬁcacy

pharmacovigilance studies, is a persistent concern. Opportunities

in a phase III multi-centre trial of the RTS,S/AS01 malaria vaccine in African

exist to overcome these challenges, such as via the use of cost-

children. Malar J 2011;10:222.

efﬁcient vaccine technology platforms for which robust safety [18] Agnandji ST, et al. First results of phase 3 trial of RTS,S/AS01 malaria vaccine in

African children. N Engl J Med 2011;365(20):1863–75.

databases already exist, as well as strengthening vaccine safety and

[19] The RTS, S.C.T.P. A Phase 3 Trial of RTS,S/AS01 Malaria Vaccine in African Infants.

pharmacovigilance systems in developing countries.

N Engl J Med 2012;594.

For vaccines targeting infection and/or clinical disease, an [20] Asante KP, et al. Safety and efﬁcacy of the RTS,S/AS01E candidate malaria vac-

absence of deﬁned biomarkers of protection, both for naturally cine given with expanded-programme-on-immunisation vaccines: 19 month

follow-up of a randomised, open-label, phase 2 trial. Lancet Infect Dis

acquired immunity and for proven vaccine approaches, remain sig-

2011;11(10):741–9.

niﬁcant challenges that, if overcome, could signiﬁcantly accelerate

[21] Agnandji ST, et al. Evaluation of the safety and immunogenicity of the

development of next-generation vaccines. For vaccines targeting RTS,S/AS01E malaria candidate vaccine when integrated in the expanded pro-

gram of immunization. J Infect Dis 2011;202(7):1076–87.

transmission, deﬁnition of a clear regulatory approach represents

[22] Garcon N, Chomez P, Van Mechelen M. GlaxoSmithKline Adjuvant Systems

the most apparent and immediate challenge that will need to be

in vaccines: concepts, achievements and perspectives. Expert Rev Vaccines

overcome to enable a clear development pathway. 2007;6(5):723–39.

[23] Global Vaccine Safety Blueprint Project, http://www.who.int/immunization

Over the coming years, the availability of a revised vision and

safety/activities/GVS blueprint project/en/index.html; 2012.

strategic goals for malaria vaccine community will be of critical

[24] Roestenberg M, et al. Protection against a malaria challenge by sporozoite inoc-

importance to focus product development efforts and align them ulation. N Engl J Med 2009;361(5):468–77.

[25] Kester KE, et al. Randomized, double-blind, phase 2a trial of falciparum malaria

with global malaria control and elimination efforts. A particularly

vaccines RTS,S/AS01B and RTS,S/AS02A in malaria-naive adults: safety, efﬁcacy,

important next step will be the development of TPPs to further

and immunologic associates of protection. J Infect Dis 2009;200(3):337–46.

guide vaccine development efforts. [26] Epstein JE, et al. Safety and clinical outcome of experimental challenge

of human volunteers with Plasmodium falciparum-infected mosquitoes: an

update. J Infect Dis 2007;196(1):145–54.

Acknowledgements

[27] Sauerwein RW, Roestenberg M, Moorthy VS. Experimental human chal-

lenge infections can accelerate clinical malaria vaccine development. Nat Rev

Immunol 2011;11(1):57–64.

This work is funded in whole or part by a grant from the Bill &

[28] Hoffman SL, et al. Protection of humans against malaria by immunization

Melinda Gates Foundation. The views expressed herein are solely

with radiation-attenuated Plasmodium falciparum sporozoites. J Infect Dis

those of the authors and do not necessarily reﬂect the views of 2002;185(8):1155–64.

[29] Clyde DF. Immunity to falciparum and vivax malaria induced by irradiated

the Bill & Melinda Gates Foundation, or the position or policies of

sporozoites: a review of the University of Maryland studies, 1971–75. Bull

the World Health Organization (WHO) and International Center for

World Health Organ 1990;68(Suppl.):9–12.

Genetic Engineering and Biotechnology (ICGEB). [30] Epstein JE, et al. Live attenuated malaria vaccine designed to protect through

hepatic CD8(+) T cell immunity. Science 2011;334(6055):475–80.

[31] Stoute JA, et al. A preliminary evaluation of a recombinant circumsporozoite

Conﬂict of interest

protein vaccine against Plasmodium falciparum malaria. RTS,S Malaria Vaccine

Evaluation Group. N Engl J Med 1997;336(2):86–91.

[32] Roestenberg M, et al. Long-term protection against malaria after exper-

Chetan Chitnis is the recipient of funding from PATH MVI to

imental sporozoite inoculation: an open-label follow-up study. Lancet

support the development of a P. vivax blood stage vaccine approach. 2011;377(9779):1770–6.

David Kaslow holds stock and stock options in Merck & Co., Inc. [33] Dunachie SJ, et al. A clinical trial of prime-boost immunisation with the candi-

date malaria vaccines RTS S/AS02A and MVA-CS. Vaccine 2006;24(15):2850–9.

Christian Loucq holds stock in GlaxoSmithKline group.

[34] Laurens MB, et al. A consultation on the optimization of controlled human

malaria infection by mosquito bite for evaluation of candidate malaria vaccines.

References Vaccine 2012;30(36):5302–4.

[35] Roestenberg M, et al. Controlled human malaria infections by intradermal injec-

tion of cryopreserved Plasmodium falciparum sporozoites. Am J Trop Med Hyg

[1] WHO. World Malaria Report 2011, http://www.who.int/malaria/world malaria 2013;88(1):5–13.

report 2011/en/

[36] Herrera S. Case report: successful sporozoite challenge model in human vol-

[2] malERA. A Research Agenda for Malaria Eradication, http://www.

unteers with Plasmodium vivax strain derived from human donors. Am J Trop

ploscollections.org/malera2011; 2011.

Med Hyg 2009;81(5):740–6.

[3] Malaria Vaccine Technology Roadmap, http://www.malariavaccineroadmap.

[37] Herrera S, et al. Consistent safety and infectivity in sporozoite challenge model

net/pdfs/Malaria Vaccine TRM Final.pdf; 2006.

of Plasmodium vivax in malaria-naive human volunteers. Am J Trop Med Hyg

[4] Roberts L, Enserink M. Malaria. Did they really say. . .eradication? Science

2007;318(5856):1544–5. 2011;84(2 Suppl.):4–11.

[38] Bennett JW, Ockenhouse CF YA. Phase 1/2a open-label dose safety, reactogen-

[5] Alonso PL, et al. A research agenda to underpin malaria eradication. PLoS Med

2011;8(1):e1000406. icity, immunogenicity and efﬁcacy of the candidate Plasmodium vivax malaria

protein 001 (VMP001) administered intramuscularly with GSK Biologicals’

[6] The malERA Consultative Group on Vaccines. A research agenda for malaria

Adjuvant System AS01B in healthy malaria-naïve adults. In: ASTMH Annual

eradication: vaccines. PLoS Med 2011;8(1):e1000398.

Meeting (Abstract #953). 2011.

[7] Malaria Vaccine Technology Roadmap: update of vision and strategic goal

[39] Romero P, et al. Cloned cytotoxic T cells recognize an epitope in the circum-

underway, http://www.who.int/vaccine research/diseases/malaria/roadmap

sporozoite protein and protect against malaria. Nature 1989;341(6240):323–6.

update jul12/en/index.html; 2012.

[40] Stewart VA, et al. Priming with an adenovirus 35-circumsporozoite protein (CS)

[8] Brooks A, et al. Aligning new interventions with developing country health

vaccine followed by RTS S/AS01B boosting signiﬁcantly improves immuno-

systems: target product proﬁles, presentation, and clinical trial design. Glob

genicity to Plasmodium falciparum CS compared to that with either malaria

Public Health 2012;7(9):931–45.

vaccine alone. Infect Immun 2007;75(5):2283–90.

[9] http://www.who.int/vaccine research/links/Rainbow/en/index.html,

W.F.M.V.R.T. [41] Miyahira Y, et al. Recombinant viruses expressing a human malaria antigen can

elicit potentially protective immune CD8+ responses in mice. Proc Natl Acad

[10] Schwartz L, et al. A review of malaria vaccine clinical projects based on the

Sci U S A 1998;95(7):3954–9.

WHO rainbow table. Malar J 2012;11(11).

[42] Workshop on Heterologous Prime-Boost Strategies for HIV, Malaria and Tuber-

[11] Schwartz L, et al. A review of malaria vaccine clinical projects based on the

culosis Vaccines, http://www.who.int/vaccine research/diseases/malaria/

WHO rainbow table. Malar J 2012;11:p11.

niaid workshop hiv malaria tb 2012/en/index.htmlNIAID/WHO; 2012:

[12] Cohen J, et al. The malaria vaccine candidate RTS,S/AS is in phase III clinical

Washington DC, USA.

trials. Ann Pharm Fr 2010;68(6):370–9.

[43] Cech PG, et al. Virosome-formulated Plasmodium falciparum AMA-1 & CSP

[13] Leach A, et al. Design of a phase III multicenter trial to evaluate the efﬁcacy of

derived peptides as malaria vaccine: randomized phase 1b trial in semi-

the RTS,S/AS01 malaria vaccine in children across diverse transmission settings

immune adults & children. PLoS ONE 2011;6(7):e22273.

in Africa. Malar J 2011;10:224.

[44] Gardner MJ, et al. Genome sequence of the human malaria parasite Plasmodium

[14] Vekemans J, Leach A, Cohen J. Development of the RTS,S/AS malaria candidate

falciparum. Nature 2002;419(6906):498–511.

vaccine. Vaccine 2009;27(Suppl. 6):G67–71.

A.J. Birkett et al. / Vaccine 31S (2013) B233–B243 B243

[45] Carlton JM, et al. Comparative genomics of the neglected human malaria par- [59] Douglas AD, et al. The blood-stage malaria antigen PfRH5 is susceptible to

asite Plasmodium vivax. Nature 2008;455(7214):757–63. vaccine-inducible cross-strain neutralizing antibody. Nat Commun 2011;2.

[46] Mlambo G, Kumar N. Transgenic rodent Plasmodium berghei parasites as tools Article number 601.

for assessment of functional immunogenicity and optimization of human [60] Duncan CJ, Hill AV, Ellis RD. Can growth inhibition assays (GIA) predict

malaria vaccines. Eukaryot Cell 2008;7(11):1875–9. blood-stage 701 malaria vaccine efﬁcacy? Hum Vaccin Immunother 2012;

[47] Doolan DL, et al. Identiﬁcation of Plasmodium falciparum antigens by anti- 8(6).

genic analysis of genomic and proteomic data. Proc Natl Acad Sci U S A [61] Salanti A, et al. Evidence for the involvement of VAR2CSA in pregnancy-

2003;100(17):9952–7. associated malaria. J Exp Med 2004;200(9):1197–203.

[48] Pinder M, et al. MALVAC 2009: progress and challenges in development of [62] Trimnell AR, et al. Global genetic diversity and evolution of var genes asso-

whole organism malaria vaccines for endemic countries, 3–4 June 2009, Dakar, ciated with placental and severe childhood malaria. Mol Biochem Parasitol

Senegal. Vaccine 2009;28(30):4695–702. 2006;148(2):169–80.

[49] Sattabongkot J, et al. Establishment of a human hepatocyte line that supports [63] Ditlev SB, et al. Identiﬁcation and characterization of B-cell epitopes in the

in vitro development of the exo-erythrocytic stages of the malaria parasites DBL4epsilon domain of VAR2CSA. PLoS ONE 2012;7(9):e43663.

Plasmodium falciparum and P. vivax. Am J Trop Med Hyg 2006;74(5):708–15. [64] Gething PW, et al. A long neglected world malaria map: Plasmodium vivax

[50] Bouharoun-Tayoun H, et al. Mechanisms underlying the monocyte-mediated endemicity in 2010. PLoS Negl Trop Dis 2012;6(9):pe1814.

antibody-dependent killing of Plasmodium falciparum asexual blood stages. J [65] Gething PW, et al. A new world malaria map: Plasmodium falciparum endemi-

Exp Med 1995;182(2):409–18. city in 2010. Malar J 2011;10:p378.

[51] Persson C, et al. Cutting edge: a new tool to evaluate human pre-erythrocytic [66] Ceesay SJ, et al. Changes in malaria indices between 1999 and 2007 in The

malaria vaccines: rodent parasites bearing a hybrid Plasmodium falciparum Gambia: a retrospective analysis. Lancet 2008;372(9649):1545–54.

circumsporozoite protein. J Immunol 2002;169(12):6681–5. [67] O‘Meara WP, et al. Effect of a fall in malaria transmission on morbidity and

[52] Thompson J, et al. What can transgenic parasites tell us about the mortality in Kiliﬁ, Kenya. Lancet 2008;372(9649):1555–62.

development of Plasmodium-speciﬁc immune responses? Parasite Immunol [68] Smith DL, et al. Revisiting the basic reproductive number for malaria and its

2008;30(4):223–33. implications for malaria control. PLoS Biol 2007;5(3):pe42.

[53] HIV Vaccine Trials Network (HVTN) Laboratory, http://labs.fhcrc.org/mcelrath/ [69] Wu Y, et al. Phase 1 trial of malaria transmission blocking vaccine candi-

HVTN Lab page/index.html dates Pfs25 and Pvs25 formulated with montanide ISA 51. PLoS ONE 2008;3(7):

[54] Thera MA, et al. A ﬁeld trial to assess a blood-stage malaria vaccine. N Engl J e2636.

Med 2011;365(11):1004–13. [70] Malkin EM, et al. Phase 1 vaccine trial of Pvs25H: a transmission

[55] Bang G, et al. Pre-clinical assessment of novel multivalent MSP3 malaria vaccine blocking vaccine for Plasmodium vivax malaria. Vaccine 2005;23(24):

constructs. PLoS ONE 2011;6(12):pe28165. 3131–8.

[56] Genton B, et al. A recombinant blood-stage malaria vaccine reduces Plasmodium [71] Ockenhouse CF, et al. Phase I/IIa safety, immunogenicity, and efﬁcacy trial of

falciparum density and exerts selective pressure on parasite populations in a NYVAC-Pf7, a pox-vectored, multiantigen, multistage vaccine candidate for

phase 1-2b trial in Papua New Guinea. J Infect Dis 2002;185(6):820–7. Plasmodium falciparum malaria. J Infect Dis 1998;177(6):1664–73.

[57] Moorthy VS, et al. Report of a consultation on the optimization of clinical chal- [72] Penny M, Maire N, Studer A, Schapira A, Smith T. What shouldvaccine deve-

lenge trials for evaluation of candidate blood stage malaria vaccines, 18–19 lopers ask? Simulation of the effectiveness of malaria vaccines. PLoS ONE

March 2009, Bethesda, MD, USA. Vaccine 2009;27(42):5719–25. 2008;3(9):3193.

[58] Tham WH, Healer J, Cowman AF. Erythrocyte and reticulocyte binding-like

proteins of Plasmodium falciparum. Trends Parasitol 2012;28(1):23–30.