We performed a sensitivity analysis for infants who were exclusively breastfed in the first few days of life (n = 106), and found that pre-F IgA prior to infection did not differ significantly between cases and controls (1.7 [1.1–2.2] log10 ng/mL vs 1.9 [0.1–2.1] log10 ng/mL; P = .85), but did differ for pre-F IgG (1.3 [1.1–1.6] log10 ng/mL vs 1.6 [1.3–1.8] log10 ng/mL; P = .01). Here, CD79A is linked to infection.