Longitudinal Severe Acute Respiratory Syndrome Coronavirus 2 Vaccine Antibody Responses and Identification of Vaccine Breakthrough Infections among Healthcare Workers Using Nucleocapsid Immunoglobulin G

Background. Long-term studies of vaccine recipients are necessary to understand severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibody durability and assess the impact of booster doses on antibody levels and protection from infection. The identification of vaccine breakthrough infections among fully vaccinated populations will be important in understanding vaccine efficacy and SARS-CoV-2 vaccine escape capacity. Methods. SARS-CoV-2 spike (S) receptor-binding domain and nucleocapsid (N) immunoglobulin (Ig) G levels were measured in a longitudinal study of 1000 Chicago healthcare workers who were infection naive or previously infected and then vaccinated. Changes in S and N IgG were followed up through 14 months, and vaccine breakthrough infections were identified by increasing levels of N IgG. Results. SARS-CoV-2 S IgG antibody levels among previously infected and previously noninfected individuals decreased steadily for 11 months after vaccination. Administration of a booster 8–11 months after vaccination increased S IgG levels >2-fold beyond those observed after 2 doses, resulting in S IgG levels that were indistinguishable between previously infected and uninfected individuals. Increases in N IgG identified vaccine breakthrough infections and showed >15% breakthrough infection rates during the Omicron wave starting in December 2021. Conclusions. These results demonstrate SARS-CoV-2 antibody changes after vaccination and breakthrough infections and identify high levels of vaccine breakthrough infections during the Omicron wave, based on N IgG increases.