The study consisted of a retrospective analysis of results of RT-PCR tests performed among HCWs employed in a large university hospital, in a specialized orthopedic hospital or in other public hospitals in Bologna, Northern Italy, who were included in a surveillance program managed by the Occupational Health Unit of the university hospital.
These data were compared to those of the population of Emilia Romagna; the region in which Bologna is located: these data were obtained from the national Civil Protection Authority. 
HCWs who either experienced a close contact with a confirmed case of Covid-19 (whether a coworker or a patient) or exhibited symptoms compatible with COVID-19 (either two major symptoms, including cough, sore throat, fever, myalgia, asthenia, anosmia, ageusia, and dyspnea, or one major and two minor symptoms, including rhinorrhea, chills, arthralgia, diarrhea, conjunctivitis, and vesicular erythema) were tested for SARS-CoV-2 infection and were included in a surveillance program, that included telephone contacts for symptoms monitoring and, where required, the prescription of medications. A further group of asymptomatic HCWs were screened on a voluntary basis.
Nasopharyngeal/oropharyngeal swab samples were analyzed by RT-PCR according to the guidelines proposed by the World Health Organization . Samples were collected between 6 March 2020 and 4 April 2021.
We abstracted results for the same period on swabs provided by the Civil Protection Authority, based on aggregate data provided by the Regions coordinated by the Ministry of Health, with the support of the Civil Protection and the National Institute of Public Health, to collect timely information on the number of positive tests, deaths, hospital admissions and intensive care admissions in each Province of Italy. Tests were mainly performed on symptomatic persons and asymptomatic close contacts.
Since the data on RT-PCR tests in the regional population are available only in aggregate form, and it is not possible to link the results of multiple tests performed by the same individual, we performed the analysis on the data aggregated by week, under the assumption that it was unlikely that the same individual tested positive more than once within the same week. In other words, the number of positive tests during a week represents a good approximation of the number of subjects who tested positive during that week. We tested this assumption using the data on HCW. We abstracted, for both HCWs and the general population; the number of positive tests, the number of tests performed, and the number of people at risk, the latter was considered constant over time for each week between March 9, 2020 and April 4, 2021.
We then calculated, for each week and for both HCWs and the general population, the crude proportion of positive RT-PCR tests over total tests (p), and the crude prevalence of positive test in the population (q). We calculated the ratios of these indicators among HCWs and in the general population: φ = p(HCW) / p(pop); ψ = q(HCW) / q(pop), and analyzed how they changed over time. To this aim we conducted joinpoint analyses using the Joinpoint Regression Program of the National Cancer Institute (Joinpoint Regression Program, Version 220.127.116.11. Statistical Research and Applications Branch, National Cancer Institute, March 2021)  to identify changes in trends over time, setting the maximum number of changes to 5.
Vaccination of HCWs started at the end of December 2020. Almost all vaccinated HCWs received a double dose of the Comirnaty vaccine (BioNTech / Pfizer), while only a very small percentage in the final phase of the vaccination campaign received Moderna COVID-19 (mRNA-1273) vaccine or the Oxford / AstraZeneca COVID-19 vaccine.