Recent perspectives in clinical development of malaria vaccines

Abstract

Malaria vaccine research has progressed significantly, with RTS,S/AS01 and R21/Matrix-M receiving WHO endorsement in 2021 / 2023. These vaccines show promise, but challenges like vaccine adherence, strain variation, and resistance persist, highlighting the need for more effective, broad-reaching interventions.

The first human clinical trials of a vaccine against malaria occurred over 50 years ago in 1973¹. While some progress has been made over the last decade, the number of cases of malaria have continued to rise in endemic countries, with an estimated 263 million cases occurring in 85 regions in 2023 (https://www.who.int/publications/i/item/9789240104440). Notably, there has been a significant rise in cases in certain regions, including the Middle East, with some countries that have not reported local transmission in years now experiencing new outbreaks. Widespread transmission, linked to over 600,000 malaria deaths in 2022, is driven by factors such as climate change, geopolitical instability, and drug resistance in parasites alongside insecticide resistance in vectors. Increased temperature and humidity associated with climate change is likely to increase breeding opportunities for mosquito vectors in both endemic and naïve regions, increasing transmission (https://www.who.int/publications/i/item/9789240086173). Climate driven spread is particularly noteworthy should the parasite move into areas with little acquired immunity in the population. Additionally, political instability in regions of endemic transmission, like parts of Africa, as well as in areas with increasing transmission, such as the Middle East, is likely to limit public health initiatives, including vaccination campaigns, and vector control efforts². Finally, there is a concerning growth in insecticide resistant vectors across endemic regions, with most countries reporting resistance of the Anopheles mosquito to at least one agent³. This is further complicated by increasing prevalence of drug-resistance in Plasmodium species, limiting both treatment and prophylactic efficacy. For example, chloroquine is no longer effective against the most lethal malaria parasite strain Plasmodium (P.) falciparum in most regions, and artemisinin resistance is growing⁴. These challenges make effective development and deployment of vaccines critically important to reduce transmission, as well as the morbidity and mortality associated with malaria globally.

While it is not possible to do justice to the wide variety of vaccination approaches and target antigens studied and progressed to human trials in this short comment, we herein focus on the most recent key vaccination approaches trialled in field studies (Fig. 1), as well as highlight some potential considerations for future research.

Fig. 1. Malaria vaccine targets at different phases of parasite lifecycle.

Created in BioRender. Apostolopoulos, V. (2025) https://BioRender.com/svdsgac.

Vaccines against malaria have been a research priority for several decades, however only in 2021 did the RTS,S/AS01 (Mosquirix) vaccine receive World Health Organisation (WHO) endorsement, followed by the R21/Matrix-M vaccine in 2023. The RTS,S/AS01 vaccine is based on the RTS,S antigen, developed in 1987 by GlaxoSmithKline and the Walter Reed Army Institute of Research (WRAIR), consisting of a fragment of the circumsporozoite protein (CSP) of P. falciparum fused to the hepatitis-B surface-antigen (HBsAg), in a liposome based adjuvant mixture of the danger signal Monophosphoryl Lipid A (MPL-A) and QS-21 saponin, called AS01⁵. As the key attachment protein facilitating liver cell entry, targeting CSP aims to prevent the initial pre-erythrocytic stage of infection. Phase-III trials of RTS,S/AS01 showed a 36% reduction in clinical presentations of malaria in children 5-17 months old, and 25.9% in infants 6-12 weeks old, over four years⁶, and phase-IV monitoring by the WHO confirmed a strong safety profile and found a 9% reduction of all-cause childhood mortality, even with low uptake of the full vaccination schedule⁷. However, while promising, these efficacy rates were substantially lower than the 90% over 12 months of follow up targeted by the WHO in their preferred product characteristic guidance⁸, and full coverage of the vaccine required four doses over 14 months, a challenging vaccine-adherence target in malaria endemic regions. Addressing some of these challenges, in October 2023, the R21/Matrix-M vaccine received WHO authorisation for use and distribution. The R21/Matrix-M vaccine similarly utilises a P. falciparum CSP fragment, conjugated to HBsAg, but at a 5-fold higher CSP to HBsAg ratio. It further uses the Matrix-M adjuvant, a mixture of saponins, cholesterol and phospholipids developed by Novavax. In Phase-III trials in Africa in a clinically relevant children population, the R21/Matrix-M vaccine reduced malaria by 75% over up to 18 months in regions with seasonal transmission, and 68% over 12 months in areas with year-round malarial circulation⁹. The R21/Matrix-M vaccine is cheaper to produce and can be manufactured at a larger scale than RTS,S, a critical factor to support wide scale vaccination for low resource settings. Initial studies further show a strong safety profile. While these are excellent results, this vaccine is still limited by the requirement for three doses and a booster dose over a 14-month period, which may limit full coverage in at risk communities. It will also take time for ongoing pharmacovigilance in phase-IV monitoring to confirm its safety, efficacy and impact on disease burden, as it is rolled out to millions of children across P. falciparum endemic areas. There are no head-to-head comparisons of the two vaccines to date, and the initial studies differ too greatly to allow for reliable direct comparisons. Moreover, while effective immunity against Plasmodium parasites is associated with significant antibody titres following vaccination, natural protective immunity targeting P. falciparum CSP is associated with central memory T-cell responses¹⁰, not usually monitored during human clinical trials. The contribution and specific types of T-cells that contribute to vaccine induced protection remains an area of exploration.

While the RTS,S/AS01 and R21/Matrix-M vaccines showed it is possible to protect against malaria in clinically relevant populations, there have been, and continue to be, many other vaccines aiming to provide more effective or affordable interventions. The CSP antigen has received the most attention, given pioneering studies showing it was a target of protection by immunisation with irradiated Plasmodium sporozoites in animals and naïve human volunteers achieving an impressive 80-100% protection. Overcoming substantial logistical challenges, recent clinical trials showed 20-52% efficacy in different areas of Africa, for the leading irradiated PfSPZ or chemically attenuated PfSPZ-Cvac sporozoite vaccines¹¹. More recently, data has emerged from early trials of a late arresting genetically engineered P. falciparum sporozoites, named GA2, which have a mei2 gene deletion, which halts progression into infectious erythrocytic merozoites¹². Two early-stage trials, found strong efficacy of 90% over 6 weeks after a single dose¹³ and 89%, after three weeks following a course of three immunisations 28 days apart¹⁴. A major limitation of the whole sporozoite vaccines continues to be the requirement for liquid nitrogen storage, adding logistic complexity to the vaccine deployment. Importantly however, whole sporozoite vaccines in humans protected against heterologous P. falciparum strains, likely by the induction of immunity to other pre-erythrocytic antigens^15,16. This finding may circumvent a limitation of vaccines like RTS,S/AS01 and R21/Matrix-M, which contain one only polymorphic variant of CSP, with limited evidence of an ability to cross-react and protect against heterologous parasite strains, potentially allowing immune selection of escape variants. More recent vaccine formulations further include the thrombospondin related anonymous protein (TRAP) antigen, another surface sporozoite component¹⁷. TRAP and CSP fragments have been incorporated into diverse viral vectored vaccines used in heterologous prime-boost approaches across multiple human clinical trials, most recently a combination of CSP, AMA1 (blood stage antigen) and TRAP, showed superiority to CSP and AMA1 alone¹⁷. While results were encouraging in non-human primates and naïve humans, less positive results in Africa slowed down the progress of such viral-vector approaches. Indeed, it is notable that protection across multiple types of vaccines, including RTS,S/AS01, R21/Matrix-M, PfSPZ, PfSPZ-Cvac and viral vectored vaccines usually demonstrate higher efficacy in naïve individuals, than volunteers living in malaria endemic regions. This is likely due to a multitude of reasons, i.e., parasite polymorphism¹⁸ and presence of highly immunosuppressive regulatory T-cells in individuals harbouring malaria parasites¹⁹. A recent randomised study in Thailand treated volunteers with a combination of anti-malarials at the time of R21/Matrix-M immunisation²⁰, which could help minimise parasite mediated immunomodulation. Albeit initial analysis showed no differences in antibody responses, assessing central memory T-cells linked to malaria protection is needed for definitive conclusions.

The multistage lifecycle of plasmodium species provides many other antigenic targets for vaccines. The blood stage of the Plasmodium lifecycle, where the parasite invades erythrocytes, is being evaluated to help decrease disease severity as well as ‘mop-up’ residual parasites that circumvent pre-erythrocytic immunity. The RH5.1/Matrix-M vaccine, which targets reticulocyte-binding protein homologue 5 (RH5), part of a heteropentameric invasion complex that facilitates invasion of erythrocytes in humans, completed phase-I trial in Tanzania²¹, showing acceptable safety and reactogenicity. A more recent phase 2b trial of RH5.1/Matrix-M of 361 children, 5-17 months old in Burkina Faso, in a context of seasonal malaria showed 55% efficacy over 6 months²². This is the first study to show efficacy of a blood stage vaccine in an in-field setting of malarial transmission. Multiple other blood stage vaccine candidates have targeted antigens associated with merozoites, the unicellular parasitic organisms which exit the liver to invade erythrocytes. Antigens including AMA-1, and MSP1,2,3 have been evaluated in humans²³, however discouraging field results moved research interest away from pure blood stage vaccines. Though still emerging, transmission-blocking vaccines targeting the parasite’s sexual stages in both humans and mosquitoes are a promising addition to malaria control strategies, as they prevent the parasite from reaching the mosquito’s salivary glands, thereby interrupting transmission. The integration of these vaccines with pre-erythrocytic vaccines, which target the liver stage, and erythrocytic vaccines, which target the blood stage of parasite, creates a multifaceted approach that not only reduces parasite load in humans but also prevents the further spread of the disease through mosquito vectors, enhancing efforts toward malaria eradication²⁴.

While there has been a wealth of pre-clinical approaches on vaccine carriers, from new adjuvants to diverse types of nano and microparticles, translation to human clinical trials has been slow²⁵. Following the rapid development of mRNA and nanoparticle vaccines in the wake of the COVID-19 pandemic, there has been an increased interest in these technologies for malaria vaccines, as a more rapid and logistically more easily deployable approach. Promising preclinical research on mRNA vaccines targeting CSP and other antigens have been conducted, but no human data has yet been produced – though some candidates have entered early trials. Other recent innovative approaches include an engineered monoclonal antibody with long half-life (56-days; CIS43LS), which protected against P. falciparum infection following intravenous or subcutaneous administration²⁶. Additional efforts have been focussed on limiting the impact of malaria in pregnancy. The PAMVAC and PRIMVAC vaccine candidates, target the VAR2CSA protein, which mediates binding of P. falciparum to the placenta via chondroitin sulphate A, and have both been shown to have acceptable safety and immunogenicity in early trials^27,28, and are moving into phase I/II studies. While most research and control efforts have focused on P. falciparum, it is important to recognise that P. vivax is a leading cause of malaria in regions outside sub-Saharan Africa, including parts of Asia, Latin America, and the Middle East. Though less lethal than P. falciparum, it can cause recurrent disease due to its dormant liver stage (hypnozoites), leading to periodic relapses. Vaccine development for P. vivax lags behind P. falciparum, despite some early trials of vaccines against P. vivax showing some evidence of immunogenicity, though larger trials are required to show efficacy²⁹. Tackling P. vivax’s unique challenges, including relapse and drug resistance, is critical for effective malaria control and eradication efforts globally.

In the coming years, efforts will focus on utilising vaccine technologies capable of increasing the longevity of protection with less intensive immunisation schedules to target a range of antigens. However, rather than simply broadening the antigen scope, the emphasis should be on increasing specificity, particularly by targeting multiple conserved B/T-cell epitopes across heterologous strains, while avoiding the “distractions” of highly variable and polymorphic immunodominant regions²⁵. The existence of some conserved antigens across P. falciparum and P. vivax show further potential for the future exploration of cross-species vaccines³⁰. Moreover, while antibodies are relatively easy to measure in clinical trials, it will be important to measure T-effector and central memory T-cell immunity, to improve and refine correlates for short and long-term protective immunity.

Author contributions

J.F. was involved in the conception of the article, literature searches, analysis and interpretation of the literature, draughted the original paper, revised the paper and draughted the figure. M.P. was involved in the conception of the article, literature recommendations, analysis and interpretation of the literature, revised the paper. V.A. was involved in the conception of the article, literature searches, analysis and interpretation of the literature, revised the paper and overall supervision. J.F., M.P. and V.A. approved the final version of the article.

Competing interests

The authors declare no competing interests.