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Association between the cross-reactive anti–SARS-CoV-2 S1 (A), S-RBD (B), and NP (C) antibody titers and age in prepandemic serum samples from children and adults. Dashed lines indicate the antibody positivity cutoff for anti-S1, S-RBD, and NP IgG antibodies. Abbreviations: IgG, immunoglobulin G; NP, nucleocapsid protein; RBD, receptor-binding domain; S, spike; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2.
