Vaccines to Interrupt Malaria Transmission: Discovery and Assays

National Institute Of Allergy And Infectious Diseases

Investigators

Linked publications, trials & patents

Abstract

From our 2025 publication, we report the following progress to improve vaccine platforms and assays: Healy SA, Sagara I, et al. A vaccine that blocks Plasmodium falciparum transmission. 2025. NEJM Evidence. Jul;4(7):EVIDoa2400188. doi: 10.1056/EVIDoa2400188. Epub 2025 Jun 24. In 2023, we reported progress with leading malaria transmission–blocking vaccine candidates Pfs25 and Pfs230D1 formulated in Alhydrogel, showing safety, immunogenicity and confirming superior functional activity and durability of responses to Pfs230D1 over Pfs25 in Malian adults. However, we were unable to assess vaccine effectiveness likely due to low event rates, and hypothesized Pfs230D1 activity could be improved by the more potent adjuvant AS01 (the adjuvant administered with licensed vaccine RTS,S), which improved durability of Pfs25 and Pfs230D1 antibody responses in preclinical studies. In this 2025 publication, we report a randomized, double-blind, comparator-controlled Phase 1 trial that evaluated two Pfs230D1-EPA regimens, each formulated in AS01, on a 0-, 1-, 4-, 16-month schedule: Pfs230D1-full (consisting of 40 μg of Pfs230D1-EPA plus 50 μg of AS01 for each dose) vs. Pfs230D1-fractional (identical to Pfs230D1-full except the third dose used 8 μg of Pfs230D1-EPA and 10 μg of AS01). Pfs230D1-EPA/AS01 regimens generated antibody responses and functional activity that persisted for up to 1 year post-vaccination. We demonstrated the full dosing regimen was associated with lower transmission of naturally circulating parasites to mosquitoes in vivo across two malaria seasons. Speciation of oocysts in mosquitoes showed that Pfs230D1 vaccination was associated with markedly reduced P. falciparum transmission. The results of this trial demonstrate a significant improvement over our previous Pfs230D1-EPA formulated in Alhydrogel® in the same population of Malian adults, by formulating the antigen in the more potent adjuvant AS01. Importantly, it established feasibility and viability of the direct skin feeding bioassay to measure vaccine efficacy as the reduction of parasite transmission to mosquitoes. This publication reports the first demonstration of a transmission-blocking vaccine to reduce the proportion of infected mosquitoes (by 77.3%), marking a major advance for malaria transmission-blocking vaccines, and providing evidence that the DSF bioassay may be used to measure surrogate efficacy endpoints for TBV in future phase 2 and phase 3 trials. In FY2025 unpublished work: 1. We sought to develop a second-generation transmission-blocking vaccine (TBV) based on Pfs230, exploring the expression of larger fragments of Pfs230 in an optimized mammalian cell system. We discovered only fragments containing Pfs230D1 were successfully expressed, indicating that Pfs230D1 might possess chaperone activity to aid in the folding of other 6C domains. By fusing Pfs230D1 upstream of three different 6C domains from Pfs230 or Pfs48, we facilitated expression of these downstream domains, leading to a 2 to 3-fold improvement in serum activity. This study establishes a foundation for a next-generation vaccine that harnesses the activity of Pfs230D1 and incorporates additional domains to enhance overall vaccine functionality. Similar studies are now underway for the P. vivax orthologue and vaccine candidate antigen, Pvs230. 2. We generated a comprehensive methodology report describing our 15+ years of experience conducting direct skin feeding (DSF) assays at the Malaria Research and Training Center in Bamako, Mali, summarizing safety and endpoint data from over 37,000 assays conducted. This manuscript represents the largest body of data compiled on DSF and is nearing submission. 3. We developed a functional ELISA based on the epitope map of Pfs230D1. Antibodies that target Pfs230D1 recognize 2 faces of the antigen: a functional face that is exposed in the native antigen or a non-functional face that is occluded by downstream domains, these antibodies do not have functional activity. We prepared an antibody fragment (FAb) that targets the non-functional face of Pfs230D1 and developed a sandwich ELISA that captures Pfs230D1 to the ELISA plate by the non-functional face. Using purified monoclonal antibodies, we demonstrated that capturing Pfs230D1 by the non-functional face blocks non-functional monoclonal antibodies from binding, while allowing functional monoclonal antibodies to bind. We performed pilot experiments with subjects from Pfs230D1-EPA/AS01 trial in Mali and demonstrated the functional ELISA results predict transmission reducing activity better than standard ELISA. 4. To understand the optimal number of repeats of each PvCSP allele to include in a chimeric vaccine, we designed and produced a series of constructs that varied the repeat number and produced these constructs in our optimized HEK293 expression system. These included a smaller construct that contained 4 of each repeat, a larger construct with 16 of each repeat, and facsimile of the sequence that has been tested in phase 1 clinical trials (9 VK210 repeats and 1 VK247 repeat). All three of these constructs were compared in a small animal challenge model to the Pichia Pastoris-expressed construct containing 8 of each repeat. In addition, the PvCSP amino acid sequence was redesigned to display the repeat regions in a different manner. Three constructs were designed (one for VK210 variant, one for VK247 variant and a chimera that had repeats from both alleles) and expressed in HEK293 cells. All 3 constructs expressed well and the VK210 and chimera were scaled up to generate milligram quantities for small animal studies. 5. In an ongoing collaboration with International Vaccine Institute, S. Korea, to develop a multi-valent vaccine targeting malaria and iNTS (invasive Non-Typhoidal Salmonella), a number of polysaccharide–protein conjugates were synthesized using polysaccharides from iNTS and Pfs230D1 and PfCSP malaria antigens. Initial mouse studies showed enhancement of immune response against the polysaccharide and protein antigens on conjugation. Additional studies using various combinations of these conjugates showed they can be combined to target malaria and iNTS simultaneously by a single vaccine. Further optimization of these combinations is planned. A provisional patent application is filed by IVI to cover the identified combination with LMIV scientists as co-inventors. 6. We initiated multiple mosquito feeding assays (DSF, DMFA, concentrated DMFA) on human volunteers undergoing experimental P. vivax infections at the NIH CC. To date, successful transmission has been demonstrated in all 6 P. vivax inoculated participants. In addition, as part of this project, we developed a gametocyte concentration methodology to maximize transmission endpoints and have demonstrated near 100% prevalence of infection with over 20,000 P. vivax sporozoites in mosquitoes.