LAUSR

Longitudinal serology studies can assist in analyzing kinetics of antibodies to the SARS-CoV-2 virus, helping to inform public health decision-making. Our study aims to characterize circulating antibody trends over 18 months in vaccinated participants with and without evidence of COVID-19 infection. A cohort of healthcare workers employed at Boston Medical Center were followed, collecting serum samples and survey data over six timepoints from July 2020 through December 2021 (n = 527). History of SARS-CoV-2 infection, vaccination and booster status were confirmed, where possible, using electronic medical records. Serum was assessed for the qualitative detection of anti-N IgG and the semi-quantitative detection of anti-S IgG antibody levels. Piecewise regression models were utilized to characterize the antibody kinetics overtime. Anti-S IgG titers remained above the positivity threshold following infection and/or vaccination throughout the 18-month follow-up. Among participants with no evidence of COVID-19 infection, titers declined significantly faster in the initial 90 days post-full vaccination (β=-0.056) from December 2020 to March 2021 compared to the decline observed following booster dose uptake (β=-0.023) (p < 0.001). Additionally, COVID-19 infection prior to vaccination significantly attenuated the decline of anti-S IgG when compared to the infection-naïve participants following vaccine uptake (p < 0.001). Lastly, fewer boosted participants contracted Omicron (12.7%) than fully vaccinated (17.6%). Among the Omicron-positive participants, anti-S IgG titers were lower, but not significantly, than those who did not test positive when comparing by vaccination status. These findings provide novel 18-month kinetics of anti-S IgG antibodies and further highlight the durability of hybrid immunity, underlining the strong humoral response stimulated by combined infection and vaccination.