SARS-CoV-2 vaccine breakthrough infections by Omicron and Delta variants in healthcare workers

An observational prospective cohort study was conducted in vaccinated employees with acute SARS-CoV-2 infection between October 2021 and February 2022. Serological and molecular testing was performed to determine SARS-CoV-2 viral load, lineage, antibody levels, and neutral-ising antibody titres. A total of 571 (9.7%) employees experienced SARS-CoV-2 breakthrough infections during the enrolment period, of which 81 were included. The majority (n=79, 97.5%) was symptomatic and most (n=75, 92.6%) showed Ct-values < 30 in RT-PCR assays. Twenty-four (30%) remained PCR-positive for > 15 days. Neutralizing antibody titres were strongest for the wildtype, intermediate for Delta and lowest for Omicron variants. Omicron infections occurred at higher anti-RBD-IgG serum levels (p= 0.00001) and showed a trend for higher viral loads (p=0.14, median Ct-difference 4.3, 95% CI [-2.5-10.5]). For both variants, viral loads were signifi-cantly higher in participants with lower anti-RBD-IgG serum levels (p=0.02).