Recent clinical trials inform the future for malaria vaccines

Malaria vaccines are urgently needed in the fight against this devastating disease that is responsible for almost half a million deaths each year. Here, we discuss recent clinical advances in vaccine development and highlight ongoing challenges for the future.


Vaccines can block infection by targeting sporozoites
Vaccines can target sporozoites to prevent or impair infection of the liver, and therefore aim to induce immunity whereby an individual is protected against infection and disease. Live-attenuated sporozoite-based vaccines have the capacity to induce sterilizing immunity (no detectable parasitemia) against CHMI in malarianaive volunteers. This includes the PfSPZ vaccine that contains radiation-attenuated sporozoites incapable of replicating. It was recently shown that an optimized PfSPZ vaccine dosage induced sterilizing immunity against CHMI in six Tanzanian adults 1 , and this dose appeared safe in African children, which supports further evaluations in younger age groups (NCT02687373) 2 . Another approach is to combine replication-intact sporozoites with antimalarial drug prophylaxis to prevent malarial illness, known as PfSPZ-CVac. PfSPZ-CVac can induce sterilizing immunity in malaria-naive volunteers, but there is no published data in malariaendemic populations 3 . A third approach is to genetically attenuate sporozoites to prevent intrahepatic development. This includes the triple-gene knockout vaccine, GAP3KO, which recently completed an efficacy trial in malaria-naive volunteers (NCT03168854). Although sporozoite-based vaccines are promising, major obstacles include the fact that currently sporozoites cannot be cultured in vitro and are therefore isolated from infected mosquito salivary glands, and they currently require direct venous inoculation of vaccine doses and storage and transportation of vaccines using liquid nitrogen.
Subunit vaccines aim to target a key antigen, or sub-region, and have the advantage of using existing production technologies and delivery using established infrastructure and systems. The most successful is the RTS,S vaccine based on a truncated form of the major sporozoite surface antigen, circumsporozoite protein (CSP). This is expressed as a virus-like particle and administered with an adjuvant (AS01), which is used to enhance the immune response to vaccination. Promising findings from a phase 3 clinical trial in >15,000 African children led to pilot implementation of a four-dose booster regimen in children aged 5 months in Ghana, Kenya, and Malawi to further evaluate vaccine safety and efficacy, which is ongoing 4 . Additional trials are underway in African children to evaluate a modified regimen where a fraction of the third dose is administered months later, which appeared to be moderately more efficacious in malaria-naive volunteers (NCT03276962) 5 . There is also interest in whether RTS,S can synergize with seasonal malaria chemoprevention (NCT03143218).
A recent advance was the development of the R21 malaria vaccine, a virus-like particle that is similar to RTS,S, but has higher density of the CSP antigen on the particle surface and is formulated with Matrix-M adjuvant 6 . Initial results from a pediatric phase 2 clinical trial in Burkina Faso demonstrated 76% efficacy against malaria over 12 months 6 . Although this was considerably higher than vaccine efficacy achieved in previous RTS,S trials 4 , R21 was administered prior to the peak malaria season and most malaria occurred in the first 6 months 6 , making direct comparisons with RTS,S efficacy difficult to interpret. Furthermore, antibody levels had declined markedly by 12 months. Participants are still being monitored to determine vaccine efficacy over 2 years, and children are currently being enrolled in phase 3 clinical trials across multiple African sites (NCT04704830).
Vaccines targeting blood stages of malaria prevent clinical illness Blood-stage vaccines target the merozoite form or infected red blood cells and aim to control parasitemia and prevent clinical illness. There has been considerably less progress in achieving good efficacy for blood-stage vaccines, with only a handful of candidates evaluated in phase 2 clinical trials, and none having progressed further 7 . The most recent was RH5.1 comprised of full-length RH5 protein, an indispensable merozoite invasion ligand. In a phase 1/2a clinical trial in malaria-naive volunteers, RH5.1 with AS01 adjuvant was highly immunogenic but only resulted in a 1-day delay until threshold parasitemia levels were reached following CHMI compared with that of the unvaccinated control group 8 . An upcoming phase 1 clinical trial of RH5.1 will further evaluate safety and immunogenicity in Tanzanian adults and children (NCT04318002).
Merozoites express a multitude of surface proteins and invasion ligands, including many that are polymorphic, which makes it difficult to identify candidate antigens for subunit vaccines. In    addition, there are major knowledge gaps on specific mechanisms that mediate protective antimalarial immunity. Antibodies play a critical role, but the current reference growth inhibition assay (GIA) that measures antibody inhibitory activity has neither strongly nor consistently been associated with protection against malaria 7 . Several vaccines, including RH5.1, have generated substantial GIA activity but shown limited efficacy 8 . Achieving higher efficacy with merozoite vaccines may require harnessing multiple immunologic mechanisms. Recent studies found that antibodies can activate complement and mediate Fcγ-receptor effector functions against merozoites, which were associated with clinical protection in malaria-endemic population studies 9 . This creates a new opportunity to evaluate the targets of functional antibody responses and potentially identify new vaccine candidate antigens using these novel approaches 9 . Given the lack of success in developing blood-stage subunit vaccines, there is interest in whole parasite blood-stage vaccines, which present a large antigen repertoire to the immune system and may overcome the challenge of antigen-specific polymorphisms. The first clinical study of chemically attenuated parasite-infected red blood cells promisingly induced cellular immune responses in malaria-naive volunteers, but parasitespecific antibodies were not detected 10 . Further studies are needed on this particular vaccine approach, including improvements in vaccine production and formulation to avoid immune responses to the red blood cell membrane.
Once a merozoite infects a red blood cell and matures, the host cell undergoes extensive changes and expresses parasite-derived proteins on the red blood cell surface. One such protein, VAR2CSA, facilitates the accumulation and sequestration of parasite-infected red blood cells in the placenta. Malaria during pregnancy is, therefore, a major concern and can lead to adverse outcomes such as maternal anemia, preterm delivery, stillbirth, and low birthweight babies. Two VAR2CSA-based vaccines (PAMVAC and PRIMVAC) aimed at inhibiting placental malaria have recently completed phase 1 clinical trials and were immunogenic and had acceptable safety profiles, but are yet to be evaluated for vaccine efficacy 11,12 .

Developing vaccines that block malaria transmission
Vaccines that target the gametocyte and gamete forms aim to prevent parasite transmission to the mosquito and thereby block malaria transmission throughout a population. Although this vaccine approach does not offer any direct clinical protection in humans, blocking malaria transmission is an essential component of effective control and elimination strategies. There has been limited progress for transmission-blocking vaccines, with one candidate based on the gametocyte surface antigen, Pfs230 (Pfs230D1M), showing promise in early clinical trials and a phase 2 clinical trial is ongoing in Mali (NCT03917654) 13 .
These vaccine approaches targeting different stages of the parasite life cycle have the ability to afford protection against infection, disease, and/or prevent parasite transmission among a population. Although each approach is of value, there may be additional benefits to developing multi-stage subunit vaccines that incorporate antigens expressed at different developmental stages.

Future directions and opportunities
This is an exciting time for malaria vaccine development, with a number of promising clinical trials recently completed and others currently underway, although there are still considerable challenges that stand in the way of achieving vaccine goals within the next decade with several priority areas for research (Fig. 2). Malaria exposure may drive the lower vaccine responses typically observed in malaria-exposed populations compared with malarianaive individuals. As disease burden can be substantial among adolescents and adults in some populations, clinical trials to evaluate vaccine efficacy in these groups will also be needed. A persisting challenge for malaria vaccines is achieving sustained protective efficacy as antibody and cellular responses are often short-lived and wane within a year 6 . The immunological factors underlying these vaccine limitations remain poorly understood but will be crucial for achieving higher efficacy and wider vaccine implementation in endemic populations. Furthermore, very little progress has been made toward the development of vaccines against P. vivax, which needs to be accelerated.
Recent advances in vaccine technologies, including those used in licensed COVID-19 vaccines, have opened new opportunities for malaria vaccine development. This includes mRNA-based vaccine platforms of the CSP malaria antigen, which has already shown encouraging results in animal models 14 . There has also been continued interest in heterologous prime-boost with ChAd43 and Modified Vaccinia Ankara viral vectors encoding different malaria antigens alone or in combinations (e.g., AMA1, MSP1, ME-TRAP) 15 .
Further research on these areas will greatly advance the development of effective malaria vaccines in endemic populations.