Post-Vaccination SARS-CoV-2 Infections among Health Workers at the University Hospital of Verona, Italy: A Retrospective Cohort Survey

Abstract

Background: The SARS-CoV-2 vaccination campaign began on 27 December 2020 in Europe, primarily involving health workers. This study aimed to assess the SARS-CoV-2 vaccination effectiveness, as assessed by reductions in incidence, symptom severity, and further infection spreading.

Methods: A retrospective cohort study was conducted on 9811 health workers operating at the Verona University Hospital, Italy, from 27 December 2020 to 3 May 2021. All health workers were offered vaccination with Comirnaty (BNT162b2, BioNTech/Pfizer, Mainz, Germany/New York, United States), and a health surveillance program was implemented with periodical swab testing. Vaccination status and clinical data were collected using an ad hoc semi-structured questionnaire and health surveillance charts.

Results: As of 3rd of May, 82.5% of health workers had been vaccinated against SAR-CoV-2, and 177 (1.8%) had tested positive for SARS-CoV-2. Vaccination more than halved the cumulative incidence of SARS-CoV-2 infection and reduced by two-thirds the cumulative incidence of symptomatic subjects. In detail, most unvaccinated HWs were symptomatic; 50% reported fever, 45% reported ageusia/anosmia, and nearly 20% reported dyspnea. These percentages were much lower in HWs who had been vaccinated for at least 14 days (18% for fever and anosmia, 6% for dyspnea and ageusia). Moreover, cases of vaccine breakthrough were sixfold less likely to further spread the infection than unvaccinated HWs.

Conclusions: SARS-CoV-2 vaccination reduced the infection frequency among HWs, further spreading of the infection, and the presence, severity, and duration of COVID-19-related symptoms.

Keywords: COVID-19 pandemic; COVID-19 symptoms; SARS-CoV-2 vaccination; health surveillance; health workers; infection spreading.

Figure 1

Cumulative incidence of SARS-CoV-2 infections in 9811 HWs of the University Hospital of Verona, Italy, estimated by the Kaplan–Meier method. Time zero was the start of the vaccination campaign (27 December 2020) for the red curve (Group A), 14 days after the 1st dose for the blue curve (Group B), and 14 days after the 2nd dose for the green curve (Group C).

Figure 2

COVID-19-related symptoms, hospitalization, and infection spreading as a function of vaccination status.

Figure 3

Cumulative incidence of SARS-CoV-2 infections in 5804 HWs of the University Hospital of Verona, Italy, estimated by the Kaplan–Meier method. HWs not administered any vaccine (n = 1661, blue line) were compared with HWs who had received two doses at least 14 days before the 28 February 2021 (n = 4143, red line).