Study phase 1: incidence of new SARS-CoV-2 infection acquired before vaccination rollout
After exclusion of 185 HCWs with previous SARS-CoV-2 infection (n = 86) and those with the first PCR test performed after the implementation of the vaccination programme (n = 99), the study phase 1 consisted of 2,147 HCWs (Fig. 3; Study flow chart). During the 18-week period, 357 nurses acquired SARS-CoV-2 infection, which corresponds to a cumulative incidence of 16.6%.
More than half of the infected HCWs were asymptomatic (51%). The HCWs who tested positive for SARS-CoV-2 were similar in age and gender. The lowest percentage of SARS-CoV-2 infections was found among physicians (10%) and the highest among nurses (20%; p < 0.001). There was no significant difference in infection rates among HCWs working in wards involved and not involved in the treatment of COVID-19 patients or clinical and non-clinical areas, as well as across wards (Table 3 ).
Each HCW had 4 PCR tests performed as a median (IQR: 3–5 tests), with a higher number of tests performed for uninfected HCWs (p < 0.001).
Following adjustment for age, gender, and number of tests per person, there remained significant differences in the incidence of new SARS-CoV-2 infection across professional categories. Most notably, the odds of being infected were greater for nurses and lesser for physicians relative to the ‘other without patient contact’ category (OR 1.80, 95% CI 1.29–2.52, p = 0.001 and OR 0.45, 95% CI 0.30–0.68, p < 0.001, respectively; Table S2 ).