Completing the Antigen Puzzle for Malaria Vaccines: Broader Antibody Targeting, but Protection Does Not Surpass R21

After Plasmodium enters the human body, it can reach the liver within just a few hours; if even a small number of parasites successfully establish themselves, they may initiate the next stage of infection. This narrow interception window means that a malaria vaccine needs not only a large quantity of antibodies, but also antibodies capable of recognizing enough vulnerabilities. The research team therefore attempted to add two long-overlooked pieces to the antigenic landscape beyond that covered by existing vaccines.

The currently approved RTS,S and R21 vaccines both center on the circumsporozoite protein PfCSP on the surface of Plasmodium falciparum sporozoites, primarily presenting numerous NANP repeats and the protein’s carboxy terminus. However, PfCSP’s “junctional region” and “minor repeat region” are not fully included. Previous studies of human monoclonal antibodies have shown that these two regions may also serve as targets for preventing infection.

In the study published in *npj Vaccines*, researchers selected 27 from more than 100 PfCSP antigen designs for mouse challenge trials and compared them with R21 and RT-I53-50 nanoparticles displaying similar antigens. The designs named CSP X and CSP Z directed antibody responses toward the minor repeat region and junctional region, respectively. Both significantly reduced parasite burden in the livers of mice, but provided less protection than the benchmark vaccines, which primarily target the NANP repeats.

The team further placed different antigens on the same “mosaic” nanoparticle or injected mixtures of separately assembled nanoparticles. Both the mosaic formulation combining RT and CSP X and the mixed formulation of RT plus CSP X significantly reduced liver parasite burden compared with the unvaccinated group, showing that adding new epitopes alongside the existing scaffold did not compromise protection and also offered a feasible engineering route for expanding the range of antibody recognition.

However, the experiments did not demonstrate that “more targets mean stronger protection.” In the three-dose comparison, RT-I53-50 and R21 still produced the most pronounced liver protection. In a direct comparison using lower doses and a two-dose regimen, R21 also performed better across the board in both antibody levels and parasite reduction. In other words, the additional epitopes currently resemble components worth further optimization rather than an upgraded version that has already surpassed existing vaccines.

The limits of the evidence are also clear: the study conducted challenge experiments only in mice using transgenic Plasmodium expressing human PfCSP, so the results cannot be directly extrapolated to protection in humans. The pandemic caused delays in booster vaccination, and the researchers did not obtain immune data after the booster dose, making it impossible to compare the antibody breadth of the combination vaccines individually with the final liver parasite burden. The next step is to determine whether multi-epitope designs can produce durable, noncompeting antibody responses under a normal vaccination schedule and to demonstrate in primate or human studies that they provide not merely broader, but better, protection.