Antibody responses to known and unknown SARS-CoV-2 infections after mRNA vaccine booster

We followed a fully-vaccinated (two mRNA vaccine doses) community cohort (n=41) without prior COVID-19 diagnosis from September 2021 through March 2022 through the Omicron wave following a booster mRNA vaccination. 19.5% of participants reported a known SARS-CoV-2 infection during the Omicron wave, which was confirmed by anti-nucleocapsid IgG. An additional 36.5% also developed anti-nucleocapsid IgG after the boost, consistent with unknown, asymptomatic SARS-CoV-2 infection during this period. Infection defined by anti-nucleocapsid IgG, whether known to participant or not, increased anti-spike IgG levels, relative to those lacking anti-nucleocapsid IgG, at 120 days post-booster.