COVID-19 infections and effectiveness of the vaccination among healthcare workers

Introduction: New variants of the SARS-CoV-2 coronavirus are constantly appearing, causing the COVID-19 pan-demic. From November 2021, most infections were caused by the Omicron coronavirus variant.Objective: The aim of this prospective observational cohort study was to estimate the incidence of COVID-19 infec-tions in the high-risk healthcare workers after two BNT162b2 mRNA Pfizer-BioNTech vaccines and the subsequent booster vaccine, as well as the effectiveness, the safety and the humoral immune response of the vaccines.Method: We started the two Pfizer-BioNTech ((BNT162b29) vaccinations of healthcare workers of the Polyclinic of the Hospitaller Brothers of St. John between January 07 and March 08, 2021. The choice of the type and timing of the third booster vaccination was voluntary. The workers were followed up between January 07, 2021 and June 29, 2022. The infection rate, adverse events of the vaccination, risk factors to infection and the kinetics of anti-spike (S) antibody and anti-nucleocapsid (N) antibody serum level were evaluated.Results: The data of 294 healthcare workers - 96 medical doctors, 127 nurses and 71 workers in hospital - who had at least three antibody level measurements were analyzed. The third booster vaccine was given to 280 workers, the distribution of the vaccines was the following: Pfizer-BioNTech (BNT162b29) vaccine (n = 210), Moderna COVID-19 (mRNA-1273) vaccine (n = 37), Sinopharm COVID-19 vaccine (n = 21), Janssen COVID-19 (n = 10), AstraZeneca (ChAdOx1 nCoV-19) vaccine (n = 2). Infection occurred in 121 cases (41%) during the observation period. The course of the COVID-19 infections was mostly mild (97%) and recovered within a week. During the observational period, 2 workers died: a 56-year-old woman died after two vaccinations for reason unrelated to COVID-19 infection, and a 58-year-old man died after the booster vaccination, following COVID-19 infection. The incidence of infection did not correlate with age, sex, comorbidities, smoking, occupation and BMI. The me-dian titre of anti-S antibody serum level increased one month after the second vaccination of the basic immunization (1173.0 U/ml) and decreased slowly until the 8th month (678.5-625.8-538.0 U/ml) after the basic vaccination. One month after the booster vaccination, the median titre of anti-S antibody serum level increased significantly (16 535 U/ml), and showed a decreasing trend in the 3rd month after the booster vaccination (9697.7 U/ml). An exceptionally high S antibody serum level increasing after the basic (>10 000 U/mL) and booster (>60 000 U/m) vaccination showed a correlation with prior COVID-19 infection. The median cut-off index (COI) of anti-N anti-body was not affected by vaccination, the increasing of the titre is related to the infection. Conclusion: The booster vaccination had less effect on the infection caused by Omicron variant, but the course of the infection was milder. Compared to the basic immunisation, the booster vaccination caused a significant increase in the S antibody level. An exceptionally high S antibody level correlated with prior COVID-19 infection.