BackgroundAntibodies are crucial for vaccine-mediated protection against many pathogens. Modifications to vaccine delivery that increase antibody magnitude, longevity, and/or quality are therefore of great interest for maximising efficacy. We have previously shown that a delayed fractional (DFx) dosing schedule (0-1-6mo) – using AS01B-adjuvanted RH5.1 malaria antigen – substantially improves serum IgG durability as compared to monthly dosing (0-1-2mo; NCT02927145). However, the underlying mechanism and whether there are wider immunological changes with DFx dosing was unclear.MethodsImmunokinetics of PfRH5-specific Ig across multiple isotypes were compared between DFx and monthly regimen vaccinees. Peak responses were characterised in-depth with a systems serology platform including biophysical and functional profiling. Computational modelling was used to define the humoral feature set associated with DFx dosing. PfRH5-specific B cells were quantified by flow cytometry and sorted for single cell RNA sequencing (scRNA-seq). Differential gene expression between DFx and monthly dosing regimens was explored with Seurat, DESeq2 and gene set enrichment analysis.ResultsDFx dosing increases the frequency of circulating PfRH5-specific B cells and longevity of PfRH5-specific IgG1, as well as other isotypes and subclasses. At the peak antibody response, DFx dosing was distinguished by a systems serology feature set comprising increased FcRn-binding, IgG avidity, and proportion of G2B and G2S2F IgG Fc glycans, alongside decreased IgG3, antibody-dependent complement deposition, and proportion of G1S1F IgG Fc glycan. At the same time point, scRNA-seq of PfRH5-specific B cells revealed enriched plasma cell and Ig / protein export signals in the monthly dosing group as compared to DFx vaccinees.ConclusionsDFx dosing of the RH5.1/AS01B vaccine had a profound impact on the humoral response. Our data suggest plausible mechanisms relating to improved FcRn-binding (known to improve Ig longevity) and a potential shift from short-lived to long-lived plasma cells. Recent reports of the positive impact of delayed boosting on SARS-CoV-2 vaccine immunogenicity highlight the broad relevance of these data.