Bona fide multimeric secretory IgA levels were significantly higher in individuals with previous infection compared with SARS-CoV-2–naive individuals after 2 antigenic stimulations (median optical density, 0.36 [IQR, 0.16-0.63] vs 0.16 [IQR, 0.10-0.22]; P < .001). Here, CD79A is linked to infection.