FCGR2A and infection: To assess antibody features that protected from acquisition of pH1N1 infection, logistic regression models with stepwise variable selection by AIC were built for shared data from sub-study (iii) (Fig. 4) and sub-study (iv) (Fig. 5) samples, which included 4 proteins (i.e., sH1-2007, pH1, sH3-2007 and H1-stem) and 6 antibody features (i.e., IgG, IgG1, IgG3, IgA1, FcγR3a, and FcγR2a) (Supplementary Table 1), resulting in the comparison of 24 variables.