Background: The nature of protective immune responses elicited by immunization with the candidate malaria vaccine RTS,S is still incompletely understood. Antibody levels correlate with protection against malaria infection, but considerable variation in outcome is unexplained (e.g., children may experience malaria despite high anti-circumsporozoite [CS] titers).
Methods and findings: We measured the avidity index (AI) of the anti-CS antibodies raised in subgroup of 5-17 month old children in Kenya who were vaccinated with three doses of RTS,S/AS01E between March and August 2007. We evaluated the association between the AI and the subsequent risk of clinical malaria. We selected 19 cases (i.e., with clinical malaria) and 42 controls (i.e., without clinical malaria), matching for anti-CS antibody levels and malaria exposure. We assessed their sera collected 1 month after the third dose of the vaccine, in March 2008 (range 4-10 months after the third vaccine), and at 12 months after the third vaccine dose. The mean AI was 45.2 (95% CI: 42.4 to 48.1), 45.3 (95% CI: 41.4 to 49.1) and 46.2 (95% CI; 43.2 to 49.3) at 1 month, in March 2008 (4-10 months), and at 12 months after the third vaccination, respectively (p = 0.9 by ANOVA test for variation over time). The AI was not associated with protection from clinical malaria (OR = 0.90; 95% CI: 0.49 to 1.66; p = 0.74). The AI was higher in children with high malaria exposure, as measured using the weighted local prevalence of malaria, compared to those with low malaria exposure at 1 month post dose 3 (p = 0.035).
Conclusion: Our data suggest that in RTS,S/AS01E-vaccinated children residing in malaria endemic countries, the avidity of anti-circumsporozoite antibodies, as measured using an elution ELISA method, was not associated with protection from clinical malaria. Prior natural malaria exposure might have primed the response to RTS,S/AS01E vaccination.
Trial registration: ClinicalTrials.gov NCT00380393.