Low SARS-CoV-2 infection rates and high vaccine-induced immunity among German healthcare workers at the end of the third wave of the COVID-19 pandemic

In this longitudinal cohort study, we assessed the severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) seroconversion rates and analyzed the coronavirus disease 2019 (COVID-19) vaccine-induced immunity of 872 hospital workers at the University Medical Center Hamburg-Eppendorf between May 11 and May 31, 2021. The overall seroprevalence of anti–NC–SARS-CoV-2 antibodies was 4.7% (n = 41), indicating low SARS-CoV-2 infection rates and persistent effectiveness of hospital-wide infection control interventions during the second and third wave of the pandemic. In total, 92.7% (n = 808) out of the entire study cohort, 98.2% (n = 325) of those who had been vaccinated once and all 393 individuals who had been vaccinated twice had detectable anti-S1-RBD-SARS-CoV-2 antibody titers and no significant differences in vaccine-induced immune response were detected between male and female individuals and between different age groups. Vaccinated study participants with detectable anti–NC–SARS-CoV-2 antibody titers (n = 30) developed generally higher anti-S1-RBD-SARS-CoV-2 antibody titers compared to anti–NC–SARS-CoV-2 negative individuals (n = 694) (median titer: 7812 vs. 345 BAU/ml, p < 0.0001). Furthermore, study participants who received heterologous vaccination with AZD1222 followed by an mRNA vaccine showed markedly higher anti-S1-RBD-SARS-CoV-2 antibody titers than individuals who received two doses of an mRNA vaccine or two doses of AZD1222 (median titer: AZD1222/AZD1222: 1069 BAU/ml, mRNA/mRNA: 1388 BAU/ml, AZD1222/mRNA: 9450 BAU/ml; p < 0.0001). Our results indicate that infection control interventions were generally effective in preventing nosocomial transmission of SARS-CoV-2 and that COVID-19 vaccines can elicit strong humoral responses in the majority of a real-world cohort of hospital workers.