Plasmodium vivax thrombospondin related adhesion protein: immunogenicity and protective efficacy in rodents and Aotus monkeys

Plasmodium vivax vaccine: What is the best way to go?

Malaria is one of the most devastating human infectious diseases caused by Plasmodium spp. parasites. A search for an effective and safe vaccine is the main challenge for its eradication. Plasmodium vivax is the second most prevalent Plasmodium species and the most geographically distributed parasite and has been neglected for decades. This has a massive gap in knowledge and consequently in the development of vaccines. The most significant difficulties in obtaining a vaccine against P. vivax are the high genetic diversity and the extremely complex life cycle. Due to its complexity, studies have evaluated P. vivax antigens from different stages as potential targets for an effective vaccine. Therefore, the main vaccine candidates are grouped into preerythrocytic stage vaccines, blood-stage vaccines, and transmission-blocking vaccines. This review aims to support future investigations by presenting the main findings of vivax malaria vaccines to date. There are only a few P. vivax vaccines in clinical trials, and thus far, the best protective efficacy was a vaccine formulated with synthetic peptide from a circumsporozoite protein and Montanide ISA-51 as an adjuvant with 54.5% efficacy in a phase IIa study. In addition, the majority of P. vivax antigen candidates are polymorphic, induce strain-specific and heterogeneous immunity and provide only partial protection. Nevertheless, immunization with recombinant proteins and multiantigen vaccines have shown promising results and have emerged as excellent strategies. However, more studies are necessary to assess the ideal vaccine combination and test it in clinical trials. Developing a safe and effective vaccine against vivax malaria is essential for controlling and eliminating the disease. Therefore, it is necessary to determine what is already known to propose and identify new candidates.

Progress towards the development of a P. vivax vaccine

INTRODUCTION

Plasmodium vivax causes significant public health problems in endemic regions. A vaccine to prevent disease is critical, considering the rapid spread of drug-resistant parasite strains, and the development of hypnozoites in the liver with potential for relapse. A minimally effective vaccine should prevent disease and transmission while an ideal vaccine provides sterile immunity.

AREAS COVERED

Despite decades of research, the complex life cycle, technical challenges and a lack of funding have hampered progress of P. vivax vaccine development. Here, we review the progress of potential P. vivax vaccine candidates from different stages of the parasite life cycle. We also highlight the challenges and important strategies for rational vaccine design. These factors can significantly increase immune effector mechanisms and improve the protective efficacy of these candidates in clinical trials to generate sustained protection over longer periods of time.

EXPERT OPINION

A vaccine that presents functionally-conserved epitopes from multiple antigens from various stages of the parasite life cycle is key to induce broadly neutralizing strain-transcending protective immunity to effectively disrupt parasite development and transmission.

From a basic to a functional approach for developing a blood stage vaccine against Plasmodium vivax

Introduction: Numerous challenges have hampered developing an anti-malarial vaccine against the most widespread malarial parasite worldwide: Plasmodium vivax. Despite the progress achieved in studying proteins in short-term in vitro culture or in experimental models, there is still no clear method for defining which antigens or their regions should be prioritized for including them in a vaccine.Areas covered: The methods used by research groups so far which have focused on the functional analysis of P. vivax blood stage antigens have been reviewed here. A logical strategy orientated toward resolving two of the most commonly occurring problems in designing vaccines against this species has thus been proposed (i.e. the search for candidates and evaluating/ascertaining their functional role in the invasion of such molecules).Expert commentary: Advances in knowledge regarding P. vivax biology have been extremely slow. Only two key receptor-ligand interactions concerning merozoite entry to reticulocytes have been reported during the last 20 years: PvDBP1-DARC and PvRBP2b-CD71. Despite increasing knowledge about the parasite's intimate preference for its host cells, it has yet to be determined which regions of the merozoite molecules characterized to date meet the requirement of inducing protective immune responses effectively blocking heterologous parasite entry to human cells.

Antibody Responses Against Plasmodium vivax TRAP Recombinant and Synthetic Antigens in Naturally Exposed Individuals From the Brazilian Amazon

Thrombospondin-related adhesive protein (TRAP) is essential for sporozoite motility and the invasion of mosquitoes' salivary gland and vertebrate's hepatocyte and is, thus, considered a promising pre-erythrocytic vaccine candidate. Despite the existence of a few reports on naturally acquired immune response against Plasmodium vivax TRAP (PvTRAP), it has never been explored so far in the Amazon region, so results are conflicting. Here, we characterized the (IgG and IgG subclass) antibody reactivity against recombinant PvTRAP in a cross-sectional study of 299 individuals exposed to malaria infection in three municipalities (Cruzeiro do Sul, Mâncio Lima and Guajará) from the Acre state of the Brazilian Amazon. In addition, the full PvTRAP sequence was screened for B-cell epitopes using in silico and in vitro approaches. Firstly, we confirmed that PvTRAP is naturally immunogenic in the cohort population since 49% of the individuals were IgG-responders to it. The observed immune responses were mainly driven by cytophilic IgG1 over all other sublcasses and the IgG levels that was corelated with age and time of residence in the studied area (p < 0.05). Interestingly, only the levels of specific anti-TRAP IgG3 seemed to be associated with protection, as IgG3 responders presented a significantly higher time elapse since the last malaria episode than those recorded for IgG3 non-responders. Regarding the B-cell epitope mapping, among the 148 responders to PvTRAP, four predicted epitopes were confirmed by recognition of antibodies (PvTRAP_R197-H227; PvTRAP_E237-T258; PvTRAP_P344-G374; and PvTRAP_E439-K454). Nevertheless, the frequency of responders against these peptides were low and did not show a clear correlation with the antibody response against the corresponding antigen. Moreover, none of the linear confirmed epitopes were located in the binding regions of PvTRAP in respect to the host cell ligand. Collectively, our data confirm the PvTRAP immunogenicity among Amazon inhabitants, while suggesting that the main important B-cell epitopes are not linear.

Naturally acquired IgG antibodies to thrombospondin-related anonymous protein of Plasmodium vivax (PvTRAP) in Thailand predominantly elicit immunological cross-reactivity

BACKGROUND

Thrombospondin-related anonymous protein (TRAP) is a prime candidate for a malaria vaccine. Antibodies to Plasmodium vivax TRAP (PvTRAP) occur upon natural infection while specific antigenic domains remain to be addressed.

METHODS

The PvTRAP sequences were determined from 73 P. vivax isolates from Tak and Ubon Ratchathani provinces collected in 2013. The recombinant proteins representing four variants each for domain II (A domain) and domain IV (thrombospondin repeat region) of PvTRAP circulating in these areas were used as antigens in enzyme-linked immunosorbent assay against 246 serum samples from P. vivax-infected patients in both provinces collected during 2013 and 2014.

RESULTS

The prevalence of total IgG antibodies to at least one variant antigen of domain II and domain IV was 63.8% and 71.5%, respectively. Differential IgG antibody responses to these variant antigens of each domain were observed. Total IgG antibody responses to the variant antigens of each domain upon pairwise comparisons were highly correlated, suggesting immunological cross-reactivity in the majority of serum samples. A smaller proportion of serum samples contained non-cross-reactive antibodies to variants of each domain; particularly domain II in which amino acid differences significantly influenced antibody recognition. Previous malaria exposure positively affected antibody responses to domain IV. Positive seroconversion and rising antibody titres occurred within a few weeks after resolution of infections.

CONCLUSIONS

Both domains II and IV are targets of naturally acquired IgG antibodies. Despite sequence variation in these domains, most antibody responses were cross-reactive. A cross-sectional evaluation of antibodies to PvTRAP during acute infection could underestimate the seroprevalence.

Identifying Potential Plasmodium vivax Sporozoite Stage Vaccine Candidates: An Analysis of Genetic Diversity and Natural Selection

Parasite antigen genetic diversity represents a great obstacle when designing a vaccine against malaria caused by Plasmodium vivax. Selecting vaccine candidate antigens has been focused on those fulfilling a role in invasion and which are conserved, thus avoiding specific-allele immune responses. Most antigens described to date belong to the blood stage, thereby blocking parasite development within red blood cells, whilst studying antigens from other stages has been quite restricted. Antigens from different parasite stages are required for developing a completely effective vaccine; thus, pre-erythrocyte stage antigens able to block the first line of infection becoming established should also be taken into account. However, few antigens from this stage have been studied to date. Several P. falciparum sporozoite antigens are involved in invasion. Since 77% of genes are orthologous amongst Plasmodium parasites, P. vivax sporozoite antigen orthologs to those of P. falciparum might be present in its genome. Although these genes might have high genetic diversity, conserved functionally-relevant regions (ideal for vaccine development) could be predicted by comparing genetic diversity patterns and evolutionary rates. This study was thus aimed at searching for putative P. vivax sporozoite genes so as to analyse their genetic diversity for determining their potential as vaccine candidates. Several DNA sequence polymorphism estimators were computed at each locus. The evolutionary force (drift, selection and recombination) drawing the genetic diversity pattern observed was also determined by using tests based on polymorphism frequency spectrum as well as the type of intra- and inter-species substitutions. Likewise, recombination was assessed both indirectly and directly. The results showed that sporozoite genes were more conserved than merozoite genes evaluated to date. Putative domains implied in cell traversal, gliding motility and hepatocyte interaction had a negative selection signal, being conserved amongst different species in the genus. PvP52, PvP36, PvSPATR, PvPLP1, PvMCP1, PvTLP, PvCelTOS, and PvMB2 antigens or functionally restricted regions within them would thus seem promising vaccine candidates and could be used when designing a pre-erythrocyte and/or multi-stage vaccine against P. vivax to avoid allele-specific immune responses that could reduce vaccine efficacy.

Advancing Research Models and Technologies to Overcome Biological Barriers to Plasmodium vivax Control

Malaria prevalence has declined in the past 10 years, especially outside of sub-Saharan Africa. However, the proportion of cases due to Plasmodium vivax is increasing, accounting for up to 90-100% of the malaria burden in endemic regions. Nonetheless, investments in malaria research and control still prioritize Plasmodium falciparum while largely neglecting P. vivax. Specific biological features of P. vivax, particularly invasion of reticulocytes, occurrence of dormant liver forms of the parasite, and the potential for transmission of sexual-stage parasites prior to onset of clinical illness, promote its persistence and hinder development of research tools and interventions. This review discusses recent advances in P. vivax research, current knowledge of its unique biology, and proposes priorities for P. vivax research and control efforts.

What Is Known about the Immune Response Induced by Plasmodium vivax Malaria Vaccine Candidates?

Malaria caused by Plasmodium vivax continues being one of the most important infectious diseases around the world; P. vivax is the second most prevalent species and has the greatest geographic distribution. Developing an effective antimalarial vaccine is considered a relevant control strategy in the search for means of preventing the disease. Studying parasite-expressed proteins, which are essential in host cell invasion, has led to identifying the regions recognized by individuals who are naturally exposed to infection. Furthermore, immunogenicity studies have revealed that such regions can trigger a robust immune response that can inhibit sporozoite (hepatic stage) or merozoite (erythrocyte stage) invasion of a host cell and induce protection. This review provides a synthesis of the most important studies to date concerning the antigenicity and immunogenicity of both synthetic peptide and recombinant protein candidates for a vaccine against malaria produced by P. vivax.

A Library of Plasmodium vivax Recombinant Merozoite Proteins Reveals New Vaccine Candidates and Protein-Protein Interactions

Background

A vaccine targeting Plasmodium vivax will be an essential component of any comprehensive malaria elimination program, but major gaps in our understanding of P. vivax biology, including the protein-protein interactions that mediate merozoite invasion of reticulocytes, hinder the search for candidate antigens. Only one ligand-receptor interaction has been identified, that between P. vivax Duffy Binding Protein (PvDBP) and the erythrocyte Duffy Antigen Receptor for Chemokines (DARC), and strain-specific immune responses to PvDBP make it a complex vaccine target. To broaden the repertoire of potential P. vivax merozoite-stage vaccine targets, we exploited a recent breakthrough in expressing full-length ectodomains of Plasmodium proteins in a functionally-active form in mammalian cells and initiated a large-scale study of P. vivax merozoite proteins that are potentially involved in reticulocyte binding and invasion.

Methodology/Principal Findings

We selected 39 P. vivax proteins that are predicted to localize to the merozoite surface or invasive secretory organelles, some of which show homology to P. falciparum vaccine candidates. Of these, we were able to express 37 full-length protein ectodomains in a mammalian expression system, which has been previously used to express P. falciparum invasion ligands such as PfRH5. To establish whether the expressed proteins were correctly folded, we assessed whether they were recognized by antibodies from Cambodian patients with acute vivax malaria. IgG from these samples showed at least a two-fold change in reactivity over naïve controls in 27 of 34 antigens tested, and the majority showed heat-labile IgG immunoreactivity, suggesting the presence of conformation-sensitive epitopes and native tertiary protein structures. Using a method specifically designed to detect low-affinity, extracellular protein-protein interactions, we confirmed a predicted interaction between P. vivax 6-cysteine proteins P12 and P41, further suggesting that the proteins are natively folded and functional. This screen also identified two novel protein-protein interactions, between P12 and PVX_110945, and between MSP3.10 and MSP7.1, the latter of which was confirmed by surface plasmon resonance.

Conclusions/Significance

We produced a new library of recombinant full-length P. vivax ectodomains, established that the majority of them contain tertiary structure, and used them to identify predicted and novel protein-protein interactions. As well as identifying new interactions for further biological studies, this library will be useful in identifying P. vivax proteins with vaccine potential, and studying P. vivax malaria pathogenesis and immunity.

Trial Registration

ClinicalTrials.gov NCT00663546

Plasmodium vivax causes malaria in millions of people each year, primarily in Southeast Asia and Central and South America. P. vivax has a dormant liver stage, which can lead to disease recurrence in infected individuals even in the absence of mosquito transmission. The development of vaccines that target blood-stage P. vivax parasites is therefore likely to be an essential component of any worldwide effort to eradicate malaria. Studying P. vivax is very difficult as this parasite grows poorly in the laboratory and invades only small numbers of young red blood cells in patients. Due to these and other challenges, only a handful of P. vivax proteins have been tested as potential vaccines. To generate more vaccine candidates, we expressed the entire ectodomains of 37 proteins that are predicted to be involved in P. vivax invasion of red blood cells. Antibodies from Cambodian patients with P. vivax malaria recognized heat-sensitive epitopes in the majority of these proteins, suggesting that they are natively folded. We also used the proteins to screen for both predicted and novel protein-protein interactions, confirming that the proteins are functional and further supporting their potential as vaccine candidates. As a new community resource, this P. vivax recombinant protein library will facilitate future studies of P. vivax pathogenesis and immunity, and greatly expands the list of candidate vaccine antigens.

A comparative study on worldwide genetic diversity and population structure analysis of Plasmodium vivax thrombospondin-related adhesive protein (PvTRAP) and its implications for the vivax vaccine design

Plasmodium vivax thrombospondin-related anonymous protein (PvTRAP) is a promising malaria vaccine candidate; however, it exhibits sequence heterogeneity. Therefore, to design a broadly protective vivax vaccine, it is essential to have adequate information on signatures of selection and geospatial genetic diversity of global PvTRAP. For this purpose, 50 Iranian pvtrap were sequenced and compared with related available global sequences in GenBank. The nucleotide sequence analysis of Iranian pvtrap in comparison with the Sal-1 sequence showed the occurrence of 15 SNPs, and all sites were dimorphic. In total, 12 amino acid substitutions were detected and 2 of which were novel, resulting in 10 haplotypes that 8 of them were not reported in any other geographic regions. In comparison with global population, haplotype and nucleotide diversities were lowest in South Korean populations while higher levels of diversities were observed in Thai and Brazilian P. vivax populations. All 12 amino acid replacements in ectodomain of Iranian PvTRAP were distributed in predicted either B- or T-cells epitope as well as intrinsically unstructured/disordered regions (IURs). The present results revealed that observing the relatively low-level diversity in PvTRAP protein might actually be selected by immune response. In summary, the present analysis in parallel to the limited available published data has shown that genetic diversity in the global pvtrap exhibits low-level diversity and geographic variation. These results are of practical significance for the strategic development and deployment of control measures in particular for development of PvTRAP-based malaria vaccine.

Spatial variation in genetic diversity and natural selection on the thrombospondin-related adhesive protein locus of Plasmodium vivax (PvTRAP)

Thrombospondin-related adhesive protein (TRAP) of malaria parasites is essential for sporozoite motility and invasions into mosquito’s salivary gland and vertebrate’s hepatocyte; thereby, it is a promising target for pre-erythrocytic vaccine. TRAP of Plasmodium vivax (PvTRAP) exhibits sequence heterogeneity among isolates, an issue relevant to vaccine development. To gain insights into variation in the complete PvTRAP sequences of parasites in Thailand, 114 vivax malaria patients were recruited in 2006–2007 from 4 major endemic provinces bordering Myanmar (Tak in the northwest, n = 30 and Prachuap Khirikhan in the southwest, n = 25), Cambodia (Chanthaburi in the east, n = 29) and Malaysia (Yala and Narathiwat in the south, n = 30). In total, 26 amino acid substitutions were detected and 9 of which were novel, resulting in 44 distinct haplotypes. Haplotype and nucleotide diversities were lowest in southern P. vivax population while higher levels of diversities were observed in other populations. Evidences of positive selection on PvTRAP were demonstrated in domains II and IV and purifying selection in domains I, II and VI. Genetic differentiation was significant between each population except that between populations bordering Myanmar where transmigration was common. Regression analysis of pairwise linearized Fst and geographic distance suggests that P. vivax populations in Thailand have been isolated by distance. Sequence diversity of PvTRAP seems to be temporally stable over one decade in Tak province based on comparison of isolates collected in 1996 (n = 36) and 2006–2007. Besides natural selection, evidences of intragenic recombination have been supported in this study that could maintain and further generate diversity in this locus. It remains to be investigated whether amino acid substitutions in PvTRAP could influence host immune responses although several predicted variant T cell epitopes drastically altered the epitope scores. Knowledge on geographic diversity in PvTRAP constitutes an important basis for vaccine design provided that vaccination largely confers variant-specific immunity.

Natural acquisition of immunity to Plasmodium vivax: epidemiological observations and potential targets

Population studies show that individuals acquire immunity to Plasmodium vivax more quickly than Plasmodium falciparum irrespective of overall transmission intensity, resulting in the peak burden of P. vivax malaria in younger age groups. Similarly, actively induced P. vivax infections in malaria therapy patients resulted in faster and generally more strain-transcending acquisition of immunity than P. falciparum infections. The mechanisms behind the more rapid acquisition of immunity to P. vivax are poorly understood. Natural acquired immune responses to P. vivax target both pre-erythrocytic and blood-stage antigens and include humoral and cellular components. To date, only a few studies have investigated the association of these immune responses with protection, with most studies focussing on a few merozoite antigens (such as the Pv Duffy binding protein (PvDBP), the Pv reticulocyte binding proteins (PvRBPs), or the Pv merozoite surface proteins (PvMSP1, 3 & 9)) or the circumsporozoite protein (PvCSP). Naturally acquired transmission-blocking (TB) immunity (TBI) was also found in several populations. Although limited, these data support the premise that developing a multi-stage P. vivax vaccine may be feasible and is worth pursuing.

Genetic diversity and population structure of genes encoding vaccine candidate antigens of Plasmodium vivax

Background

A major concern in malaria vaccine development is genetic polymorphisms typically observed among Plasmodium isolates in different geographical areas across the world. Highly polymorphic regions have been observed in Plasmodium falciparum and Plasmodium vivax antigenic surface proteins such as Circumsporozoite protein (CSP), Duffy-binding protein (DBP), Merozoite surface protein-1 (MSP-1), Apical membrane antigen-1 (AMA-1) and Thrombospondin related anonymous protein (TRAP).

Methods

Genetic variability was assessed in important polymorphic regions of various vaccine candidate antigens in P. vivax among 106 isolates from the Amazon Region of Loreto, Peru. In addition, genetic diversity determined in Peruvian isolates was compared to population studies from various geographical locations worldwide.

Results

The structured diversity found in P. vivax populations did not show a geographic pattern and haplotypes from all gene candidates were distributed worldwide. In addition, evidence of balancing selection was found in polymorphic regions of the trap, dbp and ama-1 genes.

Conclusions

It is important to have a good representation of the haplotypes circulating worldwide when implementing a vaccine, regardless of the geographic region of deployment since selective pressure plays an important role in structuring antigen diversity.

Malaria vaccine development using synthetic peptides as a technical platform

The review covers the development of synthetic peptides as vaccine candidates for Plasmodium falciparum- and Plasmodium vivax-induced malaria from its beginning up to date and the concomitant progress of solid phase peptide synthesis (SPPS) that enables the production of long peptides in a routine fashion. The review also stresses the development of other complementary tools and actions in order to achieve the long sought goal of an efficacious malaria vaccine.

Platform for Plasmodium vivax vaccine discovery and development

Plasmodium vivax is the most prevalent malaria parasite on the American continent. It generates a global burden of 80-100 million cases annually and represents a tremendous public health problem, particularly in the American and Asian continents. A malaria vaccine would be considered the most cost-effective measure against this vector-borne disease and it would contribute to a reduction in malaria cases and to eventual eradication. Although significant progress has been achieved in the search for Plasmodium falciparum antigens that could be used in a vaccine, limited progress has been made in the search for P. vivax components that might be eligible for vaccine development. This is primarily due to the lack of in vitro cultures to serve as an antigen source and to inadequate funding. While the most advanced P. falciparum vaccine candidate is currently being tested in Phase III trials in Africa, the most advanced P. vivax candidates have only advanced to Phase I trials. Herein, we describe the overall strategy and progress in P. vivax vaccine research, from antigen discovery to preclinical and clinical development and we discuss the regional potential of Latin America to develop a comprehensive platform for vaccine development.

Conserved sequences of thrombospondin-related adhesive protein gene of Plasmodium vivax in clinical isolates from Korea

Thrombospondin-related adhesive protein (TRAP) from Plasmodium vivax (P. vivax) became one of the important vaccine candidates for malaria, because P. vivax TRAP (PvTRAP) is responsible for the sporozoite-host interactions. PvTRAP polymorphisms in the isolates from Republic of Korea (ROK) were analysed, setting the valuable baseline data for the future vaccine developments and clinical trials with PvTRAP, as a strong vaccine candidate. A total of 54 isolates were collected in 2010. PvTRAP genes from above isolates were amplified and sequenced, and the results were analysed and compared against Sal-1 strain. Sequencing analysis of 1424-bp-size PvTRAP PCR products revealed one major allelic type with six non-synonymous substitutions, where S81T, E95D, I121V and T127R substitutions were found in region II, and K371N and A425E substitutions from region IV. The ROK isolates revealed the limited sequence polymorphisms in PvTRAP in comparison with the reported isolates from other nations.

High-throughput molecular diagnosis of circumsporozoite variants VK210 and VK247 detects complex Plasmodium vivax infections in malaria endemic populations in Papua New Guinea

Malaria is endemic in lowland and coastal regions of Papua New Guinea (PNG), and is caused by Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae and Plasmodium ovale. Infection by P. vivax is attributed to distinct strains, VK210 and VK247, which differ in the sequence of the circumsporozoite protein (pvcsp). Here, based upon sequence polymorphisms in pvcsp, we developed a post-PCR ligation detection reaction-fluorescent microsphere assay (LDR-FMA) to distinguish these P. vivax strains. This diagnostic assay was designed to detect the presence of both VK210 and VK247 P. vivax strains simultaneously in a high-throughput 96-well format. Using this assay, we analyzed human blood samples from the Wosera (n=703) and Mugil (n=986) regions to evaluate the prevalence of these P. vivax strains. VK210 and VK247 strains were found in both study sites. In the Wosera, single infections with VK210 strain were observed to be most common (41.7%), followed by mixed-strain (36.8%) and VK247 single-strain infections (21.5%). Similarly, in Mugil, VK210 single-strain infections were most common (51.6%), followed by mixed-strain (34.4%) and VK247 single-strain infections (14%). These results suggest that the distribution of P. vivax infections was similar between the two study sites. Interestingly, we observed a non-random distribution of these two P. vivax strains, as mixed-strain infections were significantly more prevalent than expected in both study sites (Wosera and Mugil χ(2)p-value<0.001). Additionally, DNA sequence analysis of a subset of P. vivax infections showed that no individual pvcsp alleles were shared between the two study sites. Overall, our results illustrate that PNG malaria-endemic regions harbor a complex mixture of P. vivax strains, and emphasize the importance of malaria control strategies that would be effective against a highly diverse parasite population.

Plasmodium vivax: who cares?

More attention is being focused on malaria today than any time since the world's last efforts to achieve eradication over 40 years ago. The global community is now discussing strategies aimed at dramatically reducing malarial disease burden and the eventual eradication of all types of malaria, everywhere. As a consequence, Plasmodium vivax, which has long been neglected and mistakenly considered inconsequential, is now entering into the strategic debates taking place on malaria epidemiology and control, drug resistance, pathogenesis and vaccines. Thus, contrary to the past, the malaria research community is becoming more aware and concerned about the widespread spectrum of illness and death caused by up to a couple of hundred million cases of vivax malaria each year. This review brings these issues to light and provides an overview of P. vivax vaccine development, then and now. Progress had been slow, given inherent research challenges and minimal support in the past, but prospects are looking better for making headway in the next few years. P. vivax, known to invade the youngest red blood cells, the reticulocytes, presents a strong challenge towards developing a reliable long-term culture system to facilitate needed research. The P. vivax genome was published recently, and vivax researchers now need to coordinate efforts to discover new vaccine candidates, establish new vaccine approaches, capitalize on non-human primate models for testing, and investigate the unique biological features of P. vivax, including the elusive P. vivax hypnozoites. Comparative studies on both P. falciparum and P. vivax in many areas of research will be essential to eradicate malaria. And to this end, the education and training of future generations of dedicated "malariologists" to advance our knowledge, understanding and the development of new interventions against each of the malaria species infecting humans also will be essential.