Waning of PCV13 vaccine-induced antibody levels within the first year of life, using a 3+0 schedule: an observational population-level serosurveillance study among children under 5 years old in Blantyre, Malawi

Background. Pneumococcal conjugate vaccines (PCVs) induce serotype-specific IgG antibody, effectively reducing vaccine-serotype (VT) carriage and invasive pneumococcal disease (IPD). IgG production wanes approximately 1 month after vaccination in absence of serotype-specific exposure. With uncertainty around correlate of protection (CoP) estimates and with persistent VT carriage and VT-IPD following PCV13 introduction, we undertook population-level immunogenicity profiling among children <5 years in Blantyre, Malawi. Methods. For 638 children, capsule-specific IgG to PCV13 VTs, two non-VTs, and IgG to three pneumococcal proteins were measured using an enzyme-linked immunosorbent assay and a direct-binding electrochemiluminescence-based multiplex assay. A linear spline regression model estimated population-level, serotype-specific immunogenicity profiles. A linear regression model was used to validate putative CoPs. Findings. Immunogenicity profiles revealed a consistent pattern among VTs except serotype 3: a vaccine-induced IgG peak followed by waning to a nadir and subsequent increase in titre. For serotype 3 there was no apparent vaccine-induced increase. Heterogeneity in parameters included age range at post-vaccination-nadir (11.2 [19F, 23F] to 27.3 [7F] months). Titres dropped below IPD CoPs among 9 VTs and below carriage CoPs for 10 VTs. Study data estimated a range of carriage CoPs (0.50ug/mL to 2.50ug/mL). Increasing antibody among older children and seroincident events were consistent with ongoing VT exposure. Interpretation. A 3+0 PCV13 schedule with high uptake has not led to sustained population-level antibody immunity beyond the first year of life. Indeed, post-vaccine antibody concentrations dropped below putative CoPs for several VTs, potentially contributing to persistent VT carriage and residual VT-IPD in Malawi and other similar settings.