Serum bNAb concentrations
Each bNAb concentration in human serum was quantitatively measured using a sandwich immunoassay on Meso Scale Discovery Electrochemiluminescence (MSD-ECL) platform at PPD Bioanalytical Lab. In this assay, the bNAb was captured by biotinylated anti-idiotypic anti-bNAb antibody coated onto an MSD streptavidin plate, which was then detected by SULFO-TAG-conjugated anti-idiotypic anti-bNAb antibody. The plate was read on an MSD plate reader, resulting in assay signal proportional to the concentration of each bNAb. The method was validated following current regulatory guidance, and a sensitivity of 100 ng mWe used multiple imputation to impute missing values in serum bNAb concentrations. A detailed description can be found in Supplementary Table 