HIV Preventive Vaccines Progress to Date

Drugs 50 (5) 792-804, 1995 REVIEW ARTICLE 0012-6667/95/0011-0792/$1300/0

Jose Esparza, Saladin Osmanov and William 1. Heyward Global Programme on AIDS, World Health Organization, Geneva, Switzerland

Contents Summary , , , , , , , , , , , , , , , . . . , , . , , . , , , . , , 792 1, Scientific Challenges in the Development of HIV Vaccines , 793 1.1 What Are the Immunological Correlates of Protection? 793 1.2 What Is the Significance of HIV Genetic Variability in Relation to Vaccine Development? , . . . . . , , 794 2, Preclinical Vaccine Development ...... , ...... 795 3. Phase 1/11 Trials of HIV Preventive Vaccines .... , ...... 797 4, The Future: Prospects for the Conduct of Phase III Efficacy Trials 798

Summary The major conceptual problem for HIV vaccine development has been the lack of information on immune responses known to correlate with protection against HIV infection in humans. In this regard, studies on the natural history of HIV infection and AIDS, especially of people with apparent resistance to HIV infec tion and of patients with HIV infection who have long term survival without disease progression, may provide important information for vaccine develop ment. In addition, a major concern for the development of broadly effective vac cines has been the extensive genetic variability which is characteristic of HIY. In spite of these unknowns, the first generation of HIV candidate vaccines has been developed and evaluated. HIV candidate vaccines based on the subunit recombinant envelope concept (gp120 or gp160) have been shown to protect chimpanzees from HIV infection on challenge, and have now been evaluated in humans in phase I and phase II trials. These products are well tolerated, and capable of inducing neutralising antibodies, but not cytotoxic T lymphocytes. A second vaccine concept, currently in phase I trials, is based on live recombinant vectors, especially using poxvirus vectors followed by boosting with subunit recombinant envelope vaccines. This concept is theoretically very attractive be cause preliminary data suggest that these vaccines induce both humoral and cell mediated immunity. However, no published information is available on the ability of live recombinant vector vaccines to protect chimpanzees from HIV infection. The next step in HIV vaccine development is to proceed carefully to expanded phase II and phase III trials to assess the protective efficacy of these candidate vaccines in humans. These trials will be extremely complex from the logistical, scientific and ethical points of view, and will require close collaboration between clinical, basic science and behavioural researchers, national and international organisations, and the pharmaceutical industry. HIV Preventive Vaccines 793

More than 16 million HIV infections are nevertheless do not control the infection or devel thought to have occurred worldwide.(1,2) If the opment of AIDS. present trend continues, by the year 2000 there will Therefore, at this stage of vaccine development, be a cumulative total of 30 to 40 million HIV in it might be wise to assume that both arms of the fections worldwide, with 90% of these infections immune system may playa role in protective im occurring in developing countries. To bring this munity against HIV infection and disease, and that epidemic under control, a safe and effective pre preventive vaccines may need to induce both types ventive vaccine may be needed.(3) of immune responses. In this regard, studies on the Animal protection experiments with HIV and natural history of HIV infection and AIDS may simian immunodeficiency virus (SIV) candidate provide important information to guide further de vaccines have provided information which has velopment of HIV vaccines.(16-18) brought cautious optimism about the possibility of HIV infection is usually chronic and, in most developing an HIV vaccine. However, these pro cases, eventually leads to the development of tection experiments were conducted under ideal AIDS. It has been recently observed, however, that conditions, and their relevance to protection in hu a very small proportion of individuals apparently mans is still unknown. Moreover, information on remain uninfected despite frequent and/or exten immune correlates of protection against HIV, and sive exposure to the viruS.(19) It has been hypoth on the significance ofthe genetic variability ofHIV esised that such 'resistance' could be due to genetic in relation to vaccine efficacy, is still lacking. factors [human leucocyte antigen (HLA) haplo Since 1987 when the first HIV candidate vac type], alloimmunisation to cellular antigens or nat cines entered phase I clinical trials, it has become urally acquired immunity as a result of frequent clear that the development of an effective preven exposure to subinfectious doses of the virus.[20-26) tive vaccine will be a long and difficult process.(4- Identification of specific immune responses re 12) This brief review discusses some of the scien sponsible for such 'resistance' would provide im tific challenges that have been identified, the portant clues as to the kind of immune responses different approaches used in the development of that an effective vaccine should induce. candidate HIV vaccines, the results from ongoing In the large majority of individuals who become phase IIII clinical trials and the prospects for the infected after exposure, the initial pathogenic event conduct of large-scale phase III efficacy trials. is characterised by high-level viraemia, with infec tion of peripheral CD4+ T lymphocytes, and early 1. Scientific Challenges in the dissemination of the virus to the lymphoid sys Development of HIV Vaccines tem.[16,27) This primary viraemia is rapidly control led, at the same time when HIV-specific CD8+ cytotoxic T lymphocytes (CTLs) are generated.[28-30) 1.1 What Are the Immunological Correlates of Protection? HIV-binding antibodies are also present early dur ing primary infection, although neutralising anti The major conceptual problem for HIV vaccine bodies are detectable only after the viraemia has development is the lack of information on immune been controlled.(28,3I) responses known to correlate with protection These observations point to the important role against HIV.(ll) There has been controversy about of cell-mediated immunity in the initial anti-HIV whether a vaccine must induce humoral and/or immune response. In addition, it has been sug cell-mediated immunity for effective protec gested that low levels of viraemia may be a prog tion.(13-15) This difficulty arises because individu nostic marker for increased survival,[32-34) a situa als naturally infected with HIV develop a ' broad tion which has also been observed in the range of viral-specific immune responses, which SIV/macaque modeU35 ,36) If confirmed, this find-

ing would have important implications for the se teins, gp120 and gp41, are both derived from the lection of end-points in the design of efficacy trials cleavage of gp160, an intracellular precursor mol of HIV candidate vaccines. It is conceivable that a ecule. Neutralising antibodies are mostly directed vaccine which is not fully protective against HIV to gp120, although neutralisation epitopes have infection (incapable of providing sterilising immu also been identified in gp41 and p 17 (the matrix nity) may still decrease the initial virus load, and protein). thus delay or prevent the future course of HIV dis The principal neutralisation domain in gp120 is ease,!37-391 located within one of the 5 hypervariable regions Following the primary viraemia, the patient en of the molecule, known as the V3 loop, which in ters the asymptomatic or clinically latent period of duces the production of strain-specific neutralising infection. During this stage, the level of circulating antibodies. In addition, a number of conforma virus or virus-infected cells in peripheral blood tional epitopes are also present in gp 120, and the mononuclear cells is very low, but virus burden is neutralising antibodies which they induce are more 40 high in peripheral lymphoid tissues.l ,411 broadly reactive, some of them being capable of It is now known that HIV infection is a very preventing the binding of gp 120 to the CD4 recep dynamic process, with continuous synthesis and tor molecule. [4,5 1-561 destruction of circulating virus, accompanied by a Vaccine-induced protection may be affected by rapid turnover of CD4+ lymphocytes. There is a net the antigenic variability resulting from the genetic decrease in their number over time, and subsequent differences among envelope genes, especially if severe immunodeficiency. [42,431 this protection is dependent upon strain-specific The average duration of this asymptomatic pe neutralising antibodies. Phylogenetic analysis of riod is 8 to 10 years,[441although between 5 and 8% the nucleotide sequence of the envelope gene of of HIV-infected people may remain clinically numerous virus strains from different parts of the asymptomatic and immunologically competent for world has resulted in the identification of a number 45 up to 15 years after infection.[ 1 These 'long term of clades (or genetic subtypes) ofHIV-l,l57,581The nonprogressors' are being intensively studied in an majority of strains have been included within a ma attempt to identify possible immunological and vi jor group (group-M), which is subdivided into at rological correlates of survival, which may also be least 8 clades, designated A to H.[58-701 relevant for vaccine design. In 'addition, a number of highly divergent HIV-l More than one mechanism seems to be corre strains, which are significantly distant from group lated with long term survival, including low viral M viruses, have been identified and designated as load in blood and lymph nodes, potent and persist 'outliers', or group-O virusespl-751 Group-M vi ent cellular responses (CD8+ HIV-l-specific ruses are unequally distributed in the world, clade CTLs), persistent levels of broadly reactive neu tralising antibodies and/or infection with attenu B viruses being more prevalent in Europe, North ated strains of the virus,[32,33,45-481 which in one America and Australia; clades B, C and E in South case, but not in others, has been explained by a East Asia; and almost all clades being present in deletion in the nef gene.[49,501 different regions of sub-Saharan Africa. What is not fully understood at the present time 1.2 What Is the Significance of is the significance of genetic variability in relation HIV Genetic Variability in Relation to to the antigenic characteristics of viruses belonging Vaccine Development? to the different HIV clades.[76-781 It is interesting that most of the gp 120 conformational epitopes re HIV is characterised by extensive genetic vari lated to the CD4-binding site of clade B viruses ation, especially in the gene coding for the enve were found to be conserved among the gp120 mol lope glycoproteins (env). The 2 envelope glycopro- ecules from strains belonging to clades A to F, al-

though clade E strains were the least related to 2. Preclinical Vaccine Development clade B virusesP9] In this regard, it has been sug gested that clade Band E viruses from Thailand Two animal models have been extensively used may belong to 2 different neutralisation immuno in the development of HIV vaccines: SIV infection types.[80] On the other hand, preliminary informa in macaque monkeys, and HIV infection in chim tion obtained by investigators from the WHO Net panzees)86.87] Chimpanzees have been success work for HIV Isolation and Characterisation has fully protected with HIV experimental vaccines, suggested extensive in vitro cross-neutralisation whereas experimental SIV vaccines have been no between strains belonging to different genetic toriously poor in protecting macaque monkeys against SIV infection. This dichotomy in the ability clades (unpublished data). of different candidate vaccines to induce protective At this time, however, it is not possible to rule immunity in these 2 primate models should be kept out the possible existence of neutralisation immuno in mind when interpreting the relevance of these types, which may correspond to some of the clades experiments for humans. defined by genetic sequencing of the envelope The first protection experiments in the SIV / gene. However, a more basic question is the possi macaque model used whole inactivated virus vac ble role, if any, of neutralising antibodies in vac cines, although other studies later indicated that the cine-induced protection, and the ability of labora observed protection was due to immune responses tory tests to detect the relevant antibodies. against cellular proteins incorporated into both the The genetic diversity of HIV-I strains has also immunising and challenge virus, rather than to a been correlated with growth characteristics in vitro specific immune response against the virus.[88.89] of the virus. VIruses isolated from recently infected In addition, many did not consider the whole inac persons are usually unable to replicate in estab tivated virus vaccine as a viable approach for the lished T cell lines, or to form syncytia in peripheral development of preventive HIV vaccines, because blood mononuclear cells. These non-syncytia of the possible associated risks. Thus, the initially inducing (NSI) viruses are different from isolates developed candidate vaccines for human use were obtained from infected persons with more ad based on the subunit recombinant envelope con vanced stages of the disease, which often have a cept, an approach considered to be safer than whole syncytia-inducing (SI) phenotype)81.82] inactivated virus. These findings suggest that HIV is usually Subunit recombinant envelope candidate vac transmitted as an NSIImacrophage tropic virus, cines use gp120 or gp160 as the immunogen, and and later evolves to an SI virus form as infection are produced by genetic engineering techniques in and disease progress.[83] Of potential importance yeast, insect cells or mammalian cells. Clade B vi for vaccine development is the observation that ruses have been used in most cases.[4.5] Even NSI and SI viruses differ in their susceptibility to though the immunological correlates of HIV vac neutralising antibodies, probably due,to a different cine protection are still unknown, these vaccines configuration of the outer envelope glycoprotein. were developed with the assumption that neutralis More specifically, the principal neutralisation do ing antibodies would be an importan,t component main of gp120 (the V3100p) seems to be relatively of the protective immune response against HIY. inaccessible to V3-specific neutralising antibodies Envelope subunit recombinant vaccines pro among NSI 'clinical' isolates, whereas it is more duced in mammalian cells have been shown to accessible among 'laboratory' strains which have protect chimpanzees against HIV infection. These been adapted to continuous culture in established experiments, however, were done under well con T celllines.[84.85] Therefore, vaccines may need to trolled conditions, which included intravenous induce immune responses capable of neutralising challenge with cell-free homologous virus at the NSI viruses. peak level of postvaccination antibodies.[4] Addi-

tional experiments have shown that immunised non-envelope subunit candidate vaccines are still chimpanzees were also protected against a cell lacking. associated challenge.l90] Experiments are under Since subunit nonreplicating immunogens are way to determine if systemic immunisation with generally not efficient inducers of CD8+ CTLs, subunit recombinant envelope vaccines can protect much work has been invested in the development chimpanzees from mucosal (genital) exposure to of different approaches based on live-vectored vac HIV.l91] cines, especially using poxvirus vectors, which An important recent finding is that the neutralis would induce CTLs.l102-106] However, very few ing antibodies induced by subunit recombinant en data are available on the ability of these candidate velope vaccines in chimpanzees (as well as in hu vaccines to protect animals from HIV infection. In mans) are mostly directed against linear epitopes this regard, it has been disappointing that vaccina in the V3100p, and not conformational epitopes.[92] tion of rhesus monkeys with a recombinant vac As discussed above, these antibodies against V3 cinia virus expressing envelope and core antigens loop epitopes fail to neutralise 'clinical' isolates of was not more effective than subunit envelope pro HIV in vitro. Intriguingly, chimpanzees have also teins in conferring protection against SIV infec tion.l94] been protected from a challenge using a clinical isolate of HIV, even when the sera from the im In contrast to the relatively low protective effi cacy of SIV subunit recombinant envelope vac munised animals failed to neutralise the virus in cines, or of live vector SIV vaccines, nef-deleted vitro[91] (D. Francis, personal communication). This attenuated SIV has been reported to induce solid observation must bring some caution in the inter protection against virus challenge. [107-111] Al pretation of in vitro neutralisation tests as 'predic though the mechanisms of protection in macaque tors' of potential vaccine efficacy against clinical monkeys are not well understood, these observa isolates. tions open new possibilities for the development of Contrary to the HIV Ichimpanzee experiments, a new generation of live-attenuated vaccinesp12,1l3] vaccines based on SIV subunit envelope recombi However, since the administration of a live nant proteins have usually failed to protect mon retrovirus to an uninfected person might carry an keys against postvaccination virus challenge, al appreciable degree of risk, further research is though they have succeeded in reducing the initial needed to understand the potential risks and bene level of virus burden after infection. [36,93-95] On the fits, before decisions can be made regarding the other hand, macaques have been protected against initiation of human trials with live-attenuated HIV cell-free HIV-2 with an HIV-2 Iscom vaccine.[96] vaccines. [1 14-117] The significance of these experiments remains un Finally, one of the most exciting new develop clear, since neither the HIV/chimpanzee nor the ments in the area of vaccinology is the use of the SIVIHIV-2/macaque models have been validated direct injection of antigen-encoding free (naked) in terms of their relevance to protection in humans. DNA (nucleic acid vaccines) into the skeletal mus Since the V3 loop is the major neutralisation cle or dermal tissue, which results in protein ex domain in HIV-l, candidate vaccines based on pression and subsequent induction of antibodies multimeric V3 loop amino acid sequences have and CTLs against the encoded protein.[118-126] Re been developed.l97] Other subunit vaccines are also sults from ongoing primate protection experiments being developed based on internal proteins of the using different constructs of HIV and SIV nucleic virus, particularly the p24 core protein, which con acid vaccines are eagerly awaited, since this ap tain conserved T cell epitopes which induce cell proach may have many of the advantages of live mediated immune responses.[98-101] Animal protec attenuated vaccines, without many of its disadvan tion data with these synthetic peptides or with tages.

3. Phase 1/11 Trials of the homologous LAI strain, and to a lesser degree, HIV Preventive Vaccines against the heterologous (but also clade B virus) SF2 strain.D 28) A comparison between these im Since 1987, at least 17 candidate vaccines have mune responses, and those induced in chimpanzees been tested in small-scale phase I trials to assess which have been protected against HIV infection their safety and immunogenicity in HIV-negative by the same candidate vaccine, showed that the human volunteers; the early results ofthese candi half-life of the antibody response to gp120 was date vaccines have been reviewed elsewhere.[4,5,127) longer in humans (9 weeks) than in chimpanzees In summary, most of these candidate vaccines (2 weeks). On the other hand, immune responses have been based on the subunit recombinant enve to V3 loop epitopes, and virus neutralisation titres, lope concept, and they have been found to be gen were approximately lO-fold lower in humans than erally well tolerated in doses that are capable of in chimpanzees) 129) A similar subunit recombinant inducing HIV-specific immune responses, These envelope vaccine, but based on the more repre immune responses include binding antibodies sentative MN strain (gpI20/MN), also induced (measured by gp120- or pI60-enzyme-linked im antibodies that neutralised the homologous MN munosorbent assay, Western Blot or V3 peptide strain, as well as other clade B viruses (SF2 and binding assays), as well as production of functional LAI)) 130) antibodies with the ability to neutralise HIV infec A gp 120/SF2 candidate vaccine, combined with tivity, to mediate antibody-dependent cell cyto a novel adjuvant, MF59, and administered with or toxicity, to block gp 120-CD4 binding and to in without the immunomodulator muramyl tripeptide hibit fusion of HIV-infected cells. CD4+ CTLs, dipalmitoyl phosphatidylethanolamine (MTP-PE), and in some instances CD8+ CTLs, have been induced gpl20-binding antibodies that persisted identified in vaccine recipients, especially in those receiving live-vectored vaccines. for at least 24 weeks after a fourth injection. After 3 injections, all volunteers developed neutralising On the basis of this information, many of the candidate vaccines evaluated in the past have now antibodies against the homologous SF2 strain, been followed by newer generations that are based which in two-thirds of them also cross-neutralised on different virus strains (i.e. MN strain instead of the MN strain.[I3l) the originally used LAI strain, both of which be A fully glycosylated gp160ILAI candidate vac long to clade B), that use different manufacturing cine, combined with alum and deoxycholate adju systems (with a shift towards the use of mamma vant, also resulted in a dose-related induction of lian cells for the production of recombinant enve neutralising antibodies against the homologous lope glycoproteins), or that use newer adjuvants (in LAI strain)I32) The vaccination elicited a CTL re addition to the use of the standard aluminium hy sponse, which was not mediated by classic CD8+ droxide). Emphasis is also being put on vaccine cells, but rather by cells of the CD4+ pheno approaches which result in CTL responses. type)133) More recent results have been published from Finally, vaccination with a chimaeric gp160 phase I clinical trials of several subunit recom molecule, consisting of MN gp 120 and LAI gp41, binant envelope vaccines, all of them pro and boosting with a linear V3-MN synthetic pep duced in mammalian cells: gp120ILAI (Genentech), tide, resulted in high levels of neutralising antibod gp120/MN (Genentech), gp120/SF2 (Biocine), ies against the homologous MN strain. In about gp160/LAI (Immuno), and gp160ILAI/MN + V3 half of the volunteers, these antibodiies also neu peptides (Pasteur-Merieux). tralised the SF2 strain, but not the LAI isolate.[I34) Injection of a gp120ILAI candidate vaccine in Thus, a number of mammalian-derived subunit aluminium hydroxide adjuvant resulted in a dose recombinant envelope candidate vaccines have related induction of neutralising antibodies against been shown to be well tolerated and to induce neu-

Table I. Progress in the develol'lment of HIV-1-preventive candidate vaccines (based on published Information only) Vaccine concept Candidate vaccine Preclinical Human clinical trials (developer) studies phase I phase II Subunit nonreplicating (envelope) gp120/LAI (Genentech) .r .r gp120/MN (Genentech) .r .r .r gp120/SF2 (Bioclne) .r .r .r gp160/LAI (Immuno) .r .r gp120/MN/LAI (Pasteur-Merieux) + V3/MN .r .r synthetic peptldes V3/MN synthetic peptldes (UBI) .r .r live-recombinant vector (envelope) VaCCInia vector gp160/LAI (Bnstol-Myers Squibb) + .r .r gp160/LAI (MicroGeneSys) Canary pox vector gp160/MN/LAI + gp160/MN/LAI .r .r (Pasteur-Meneux) Whole Inactivated .r (SIV) Live attenuated .r (SIV) Nucleic aCid .r (SIV) Abbreviation: SIV = simian immunodeficiency virus tralising antibodies efficiently against the homolo ing with the gp160 subunit preparation resulted in gous strains, which in a dose-dependent fashion increasing levels of antibodies and cellular re also neutralise other strains from the same B clade. sponses.[l36-138] It is important to indicate, however, that most neu Similar prime-boost trials are being conducted tralisation experiments have been performed using with a canarypox vector encoding gp I60/MN (Pas laboratory-adapted strains of HI V-I and, as discus teur-Merieux), as well as with other poxvirus vec sed above, the antibodies were not capable of neu tors, although full results of these trials have not tralising in vitro clinical isolates of HIV- I.l92,135] yet been published. [1 34] Two of the candidate vaccines described above Similarly, data from ongoing trials with a multi (gp120 HIVIMN and gp120/SF2) are presently be meric V3 peptide vaccine (United Biomedical, ing evaluated in a phase II trial in the US among Inc.) have not been published.[l39] Candidate vac 300 healthy volunteers, some of whom are at high cines based on the nucleic acid immunisation con risk of infection.[l27] cept may enter phase I clinical trials in the near future. However, phase I testing of other vaccine Another vaccine concept which is beginning to concepts, such as using whole inactivated virus or be intensively explored is based on the use of live live-attenuated vaccines, may have to wait until ad recombinant vector vaccines, which are in most ditional preclinical data are obtained, especially in cases administered in combination with a subunit relation to their safety. recombinant envelope product. This approach aims Table I summarises the progress in the develop at the induction of both neutralising antibodies and ment of HIV candidate vaccines in relation to ani CD8+ CTLs. In fact, early clinical trials indicated mal protection experiments and phase IIII clinical that priming with a live recombinant vaccinia vec trials. tor encoding gp160 (Bristol-Myers Squibb), and boosting with a baculovirus-derived gp160 subunit 4. The Future: Prospects for the product (MicroGeneSys), resulted in stronger anti Conduct of Phase III Efficacy Trials body and cellular responses than those following either vaccine alone. These immune responses Although phase 1111 trials have shown that HIV waned after approximately 1 year, though reboost- candidate vaccines are capable of inducing HIV-l-

specific immune responses, they have not provided End-points for vaccine efficacy that are being information on the potential efficacy of the candi considered include prevention of infection, pre date vaccines because the immune correlates of vention of the establishment of chronic infection protection are not known. and the delay or prevention of progression to dis From the more than 1400 participants in phase ease. I and II trials sponsored by the National Institutes Preparation for the efficacy evaluation of HIV of Health of the US, at least 8 have acquired HIV vaccines in large-scale field trials will require ex infection during the studies.l140,141] Most of these tensive international collaboration and coordina infections occurred before the complete immunisa tion between host countries, research institutions, tion sequence was administered, and all infected pharmaceutical manufacturers, regulatory authori volunteers reported high-risk behaviour. HIV in ties, and national and international institu fections among vaccinees were expected, since the tions.[148,149] This preparation includes strengthen immunisations were given over several months, ing of research infrastructures, identification and and since no vaccine is 100% protective. Conse characterisation of potential populations for phase quently, conclusions about vaccine efficacy cannot III trials including epidemiological studies on HIV be drawn from the ongoing phase 1111 trials, but incidence,l150] social-behavioural studies to ensure large-scale phase III trials are required, involving proper counselling and informed consent,[151] viro thousands of volunteers at high risk of infec logical studies for the characterisation of prevalent tion.[142] strains in the trial population,[58] and the develop Decisions to initiate efficacy trials of HIV can ment of guidelines to ensure the conduct of the didate vaccines will be based on a careful analysis trials with the highest scientific and ethical stand of the scientific criteria, including: (i) animal pro ards,D52,153] ensuring the appropriate preparation tection data; (ii) safety and immunogenicity in hu and participation of the community. mans; (iii) information on potential correlates of It is likely that the eventual development of a protection; (iv) severity ofthe epidemic; (v) public well tolerated, highly effective and available HIV health considerations based on a r~sk/benefit anal vaccine will be a long and difficult process, requir ysis of proceeding with available candidate vac ing the development of a number of generations of cines or waiting for more data or for 'better' can vaccines over many years. In this process, it is im didate vaccines; (vi) feasibility of conducting a portant that.the initial efficacy trials are well de large-scale trial, including recruiting and retaining signed and conducted, and generate the informa the large number of volunteers needed to tion needed for the development of more demonstrate efficacy; and (vii) political and com promising vaccines in the future. munity support for the implementation of the triaUI43 ,144] References It is obvious that an analysis of the above cri 1. Merson MH. Slowing the spread ofHIV: agenda for the 1990s. teria may result in different decisions in different Science 1993; 260: 1266-8 countries.D44,1451 Some developing countries are 2. Mertens TE, Burton A, Stoneburner R, et al. Global estimates now actively preparing for phase III trials,[146,147] and epidemiology of HIV infections and AIDS. AIDS 1994; 8 Suppl. 1: S361-72 which may be initiated within the next 2 years. 3. Esparza J, Osmanov S, Kallings LO. Planning for HIV vaccine These trials will be designed to test different vac trials: the World Health Organization perspectIve. AIDS 1991; 5 Suppl. 2: S159-63 cine concepts in their ability to protect: (a) against 4. Esparza J, Osmanov S. The development and evaluatIOn ofHIV homologous and/or heterologous virus subtypes; vaccmes. Curr Opm Infect Dis 1993: 6: 218-29 (b) against different routes of transmission; and (c) 5. Walker MC, Fast PE. Clmical trials of experimental AIDS vac cmes. AIDS 1994; 8 Suppl. I: S213-36 in the presence/absence of other co-factors (e.g. 6. Haynes BE Scienlific and socia1Issues of human Immunodefi sexually transmitted diseases). ciency VIrUS vaccme development. SCIence 1993; 260: 1279-86

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