pathogens Review The Rotavirus Vaccine Landscape, an Update Roberto Cárcamo-Calvo 1, Carlos Muñoz 1, Javier Buesa 1,2 , Jesús Rodríguez-Díaz 1,2,* and Roberto Gozalbo-Rovira 1,2,* 1 Department of Microbiology, School of Medicine, University of Valencia, Av. Blasco Ibañez 17, 46010 Valencia, Spain; [email protected] (R.C.-C.); [email protected] (C.M.); [email protected] (J.B.) 2 Instituto de Investigación INCLIVA, Hospital Clínico Universitario de Valencia, 46010 Valencia, Spain * Correspondence: [email protected] (J.R.-D.); [email protected] (R.G.-R.); Tel.: +34-96-398-3316 (J.R.-D. & R.G.-R.) Abstract: Rotavirus is the leading cause of severe acute childhood gastroenteritis, responsible for more than 128,500 deaths per year, mainly in low-income countries. Although the mortality rate has dropped significantly since the introduction of the first vaccines around 2006, an estimated 83,158 deaths are still preventable. The two main vaccines currently deployed, Rotarix and RotaTeq, both live oral vaccines, have been shown to be less effective in developing countries. In addition, they have been associated with a slight risk of intussusception, and the need for cold chain mainte- nance limits the accessibility of these vaccines to certain areas, leaving 65% of children worldwide unvaccinated and therefore unprotected. Against this backdrop, here we review the main vaccines under development and the state of the art on potential alternatives. Keywords: rotavirus; gastroenteritis; vaccine; intussusception; diarrhea Citation: Cárcamo-Calvo, R.; Muñoz, 1. Background C.; Buesa, J.; Rodríguez-Díaz, J.; Rotavirus (RV) infections are the leading cause of severe acute childhood gastroenteri- Gozalbo-Rovira, R. The Rotavirus tis, accounting for the majority of diarrhea episodes among children under 6 years of age Vaccine Landscape, an Update. and representing the predominant cause of diarrhea mortality not only in humans but also Pathogens 2021, 10, 520. https:// in a wide variety of animal species [1]. Cases show higher incidence and severity between doi.org/10.3390/pathogens10050520 3 and 24 months of age [2]. Although childhood diarrhea or gastroenteritis of viral etiology has been a recognized concern throughout world history, the first human RV-associated Academic Editor: Anna Honko infection dates from 1973 [3]. In 2016, RV accounted for about 128,500 deaths worldwide, mainly in low income countries (LICs) [4]; however, the number of deaths caused by Received: 31 March 2021 this virus has decreased significantly since the introduction of RV vaccines in vaccina- Accepted: 23 April 2021 Published: 26 April 2021 tion programs from around 2008, at which time deaths amounted to 453,000 per year [5]. Other studies estimate that before vaccines were introduced, RV caused about 111 million Publisher’s Note: MDPI stays neutral episodes of gastroenteritis each year; 25 million doctor visits; two million hospitalizations; with regard to jurisdictional claims in and an average of between 352,000 and 440,000 deaths per year [6]. RV replicates in the published maps and institutional affil- enterocytes of the intestinal tract, leading to acute gastroenteritis (AGE) and shedding of iations. virus in the stool [7]. It is estimated that approximately half of RV-associated deaths occur in just four countries: India, Nigeria, Pakistan and the Democratic Republic of Congo [8]. Since 2006, The World Health Organization (WHO) has recommended including RV vaccines in vaccination programs in Europe and the Americas, extending this recommen- dation globally in 2009 [9]. By 2018, a total of 92 countries had introduced RV vaccines into Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. their national immunization programs, and in another six countries they were introduced This article is an open access article on a phased or regional basis. In the meantime, in countries with fewer resources, vaccine distributed under the terms and introduction is supported by GAVI (The Vaccine Alliance). Of 73 potential aid-receiving conditions of the Creative Commons countries, 46 introduced the vaccine in 2019 and eight more have recently been selected for Attribution (CC BY) license (https:// aid and are planning to introduce it in the future [10]. creativecommons.org/licenses/by/ Although RV incidence is greater in LICs, the virus is not limited to these countries; 4.0/). indeed, its effect is felt worldwide. Before the introduction of vaccines in the European Pathogens 2021, 10, 520. https://doi.org/10.3390/pathogens10050520 https://www.mdpi.com/journal/pathogens Pathogens 2021, 10, 520 2 of 15 Union, for example, RV was responsible for about 3.6 million episodes of AGE, 700,000 visits to the doctor; 87,000 hospitalizations; and 231 deaths among children under 5 each year, although hospitalizations have been reduced by 90% since introduction of the vaccines [11]. In addition, it is estimated that most children have been infected at least once, even if undiagnosed, by the age of 5, indicating an underestimation of the health burden caused by this virus. Similarly, although deaths caused by RV are mostly concentrated in children, RV infections can also affect adults throughout their lives, albeit with mild or even asymptomatic infections [12]. RV is a genus of the Reoviridae family and includes only viruses that infect vertebrates (birds and mammals). RV have a segmented genome composed of 11 double-stranded RNA molecules with the 30 and 50 endings preserved. The genome codes for six structural proteins (VP1-VP4, VP6 and VP7) and for six non-structural proteins (NSP1-NSP6). All segments are monocistronic, except for segment 11 which codes for two non-structural proteins (NSP5 and NSP6) [13]. The viral particles are apparently spherical, between 70 and 100 nm in diameter, with an icosahedral capsid and without a lipid envelope [14]. Infectious particles are formed by three concentric layers of protein, a structure known as a triple layered particle (TLP). The inner capsid contains the VP2 protein surrounding the genome [15] and several molecules of the VP1 proteins (the viral RNA-dependent RNA polymerase) [16] and VP3 (protein with guanylyltransferase and methylase activity) [17]; the intermediate capsid are composed of VP6 and are responsible for the main RV classification. The outer capsid also consists of VP7 protein and VP4 spicules emanating from the surface of the virion [18]. Both external capsid proteins, VP7 and VP4, possess neutralization epitopes and play an important role in virus entry and infection of target cells. In the intestine, the surface spicules, VP4, are proteolytically dissociated into two subunits, VP8* and VP5*, as a result of the presence of proteases. VP8* is responsible for specific binding to the target cell and VP5* is responsible for membrane penetration and entry into the cell [19]. During infection, a series of molecular transformations in the outer layer cleave this layer from the rest, so that only the transcriptionally active inner capsid, known as the DLP, enters the cytosol [20]. The conventional classification system is based on genome composition and immunolog- ical reactivity of three structural proteins: VP6, VP7 and VP4. Depending on the VP6 protein, they can be classified into nine species: groups A to I [14,21,22] with a recent proposal for a J species [23]. Groups A, B, C and H RV have been associated with AGE in humans and animals. The most common of the four, RVAs, are responsible for more than 90% of infections in humans and animals. Four RVA subgroups can be distinguished (SGI, SGII, SGI1II and SG nonI-nonII) based on reactivity patterns with two subgroup-specific monoclonal antibodies directed to VP6 [18]. Within these four subgroups, SGII is most frequently found in humans, while SGI is most common among those infecting animals [22]. The two external capsid proteins VP7 and VP4 trigger production of neutralizing anti- bodies, which can confer protective immunity against viral infection. Serotypes/genotypes are defined by these two outer capsid antigens: VP7 determines serotype G (a glycoprotein) and VP4 serotypes P (by sensitivity to proteases), with P[4], P[6] and P[8] being the most common. Overall, there are five prevalent genotypes: G1P[8], G2P[4], G3P[8], G4P[8] and G9P[8] [24], which represent more than 90% of strains circulating worldwide. Currently, a total of 36 G and 51 P types have been described globally [25], among which the G1P[8] genotype was the most common in the pre-vaccine era. However, after vaccine introduction the diversity of circulating genotypes has increased considerably in different countries, including South Korea, the United Kingdom and Brazil, with genotype G2P[4] reaching greater predominance [26–28]. In addition, several studies report G12P[8] as among the genotypes with most increased prevalence in the post-vaccine era [29]. Even so, it is esti- mated that the P[8] genotype is responsible for more than 80% of human infections [30]. Within this genotype, four different lineages have been described phylogenetically: P[8]- lineage I, P[8]-lineage II, P[8]-lineage III and P[8]-lineage IV [31,32]. Furthermore, recent advances in molecular characterization techniques have revealed that the P[8] genotype can be divided antigenically into different subtypes: P[8]a (composed of lineages I and III) Pathogens 2021, 10, 520 3 of 15 and P[8]b (also called OP354-like, composed of lineage IV) [33]. Most strains circulating in the world belong to subtype P[8]a, while subtype P[8]b is less common. In general, use of the terms serotype or genotype depends on whether immunological detection (MAb-ELISA and cross neutralization assay) or nucleic acid-based (sequencing, RT-PCR or oligonu- cleotide probe hybridization) methods have been used [18]. In addition to this system, there is also another one based on the genotype of the NSP4 protein, a viral enterotoxin [34].