Background BK-SE36/CpG is a lyophilized formulation of recombinant Plasmodium falciparum serine repeat antigen 5 adsorbed to aluminium hydroxide gel and reconstituted with the TLR9 adjuvant, CpG-ODN (K3). In the primary study in malaria-exposed adults and children living in Burkina Faso, the vaccine showed acceptable safety and immunogenicity profile. The 5–10 years old cohort was chosen for a follow-up study to evaluate the long-term persistency of the immune response.

Methods In the primary trial, participants were randomized to receive three doses of either BK-SE36/CpG or the control vaccine at 0, Week 4 and 16. The trial was completed in January 2020 (Day 365). The follow-up study consisted of a single visit conducted 3 years after enrolment (~24 months following D365). All children who participated in the 5–10 years-old cohort were invited for a short medical examination and a blood sample was collected for parasitological and immunogenicity measurement.

Results 44 children out the 45 in the primary trial provided consent for the follow-up study. The proportion of children with detectable anti-SE36 IgG titres in the BK-SE36/CpG arm as compared to the control arm remained high (82.8% vs. 46.7% in control group). The differences in the geometric mean of anti-SE36 IgG titers were statistically significant for vaccine arms at any timepoint: D140 (4 weeks after Dose 3: p<0.001), D365 (p<0.001) and Year 3 (p=0.039). Compared to baseline, antibody titres were 60 and 3.6 times higher at D140 and Year 3, respectively.

Conclusion BK-SE36/CpG induced high level of antibody responses in 5–10 years-old. The antibody response persisted/or remained high at Year 3. There were some indications that the antibody response can be boosted by natural infection, which may explain the sustained level of antibody titres obtained in this follow-up study.