Research Letter: Incidence of SARS-CoV-2 infection among unvaccinated US adults during the Omicron wave

As of 4/20/2022, approximately 23% of the eligible US population was unvaccinated. We studied COVID-19 infections during the Omicron (B.1.1.529) wave in unvaccinated US adults, stratified by pre-Omicron antibody levels. Anti-spike serologic testing was performed prior to the Omicron wave in the United States (9/23/21-11/5/21) and participants were surveilled to determine incident COVID-19. Only 12% of those who entered the wave with antibodies reported a test-confirmed COVID-19 infection, compared to 35% of those without antibodies prior to the Omicron wave. Effectiveness of these anti-RBD antibodies in this unvaccinated population was 67%. Among people with antibodies, titer did not appear to be associated with risk of test-confirmed Omicron infection.