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Objective: To study SARS-CoV-2 reinfections in health-care workers (HCWs) of the University Health Agency Giuliano-Isontina (ASUGI), covering the provinces of Trieste and Gorizia (northeastern Italy) routinely screened for SARS-CoV-2 via nasopharyngeal swab. Design: Cohort study of HCWs (N = 8205) followed since the start of the pandemic (1 March 2020) through 31 January 2023. The risk of reinfection during the Omicron transmission period (after 30 November 2021) among HCWs previously infected by SARS-CoV-2 was estimated based on days since last dose of COVID-19 vaccine received, adjusting for age, sex, job task, workplace, number of doses of COVID-19 vaccines and number of swab tests performed. In the crude as well as adjusted incidence rate analysis, reinfections occurring 15+ days after a first dose of COVID-19 vaccine or 8+ days following a second or more dose were counted. Results: In a highly vaccinated population, during the entire study period (1 March 2020–31 January 2023) 5253 HCWs incurred at least one SARS-CoV-2 infection, 4262 HCWs were infected only once, and 1091 were reinfected. Reinfections almost entirely (99.1% = 1071/1091) occurred after 30 November 2021, peaking in July 2022 (N = 161). Six hundred eighty-three reinfections followed a pre-Omicron primary event against 408 reinfections following an Omicron event. Reinfections during the Omicron transmission period occurred a mean of 400 ± 220 days after primary SARS-CoV-2 infection; 512 ± 205 days following a pre-Omicron primary event, as opposed to 218 ± 74 days after an Omicron primary infection. Thirty-four hospitalizations were observed, all before the Omicron wave, following 18 (0.4%) primary SARS-CoV-2 infections and 16 (1.5%) reinfections. By excluding events occurring Conclusions: SARS-CoV-2 reinfections were frequent during the Omicron transmission period, though featured by mild or no symptoms. Whilst the impact of occupation on biological risk was relatively marginal, COVID-19 vaccination had the strongest protective effect against reinfection, with a 93% VE by second booster following an Omicron primary infection.

Objective: To evaluate the incidence of primary and recurrent COVID-19 infections in healthcare workers (HCWs) routinely screened for SARS-CoV-2 by nasopharyngeal swabs during the Omicron wave. Design: Dynamic Cohort study of HCWs (N = 7723) of the University Health Agency Giuliano Isontina (ASUGI), covering health services of the provinces of Trieste and Gorizia (Northeast Italy). Cox proportional hazard model was employed to estimate the risk of primary as well as recurrent SARS-CoV-2 infection from 1 December 2021 through 31 May 2022, adjusting for a number of confounding factors. Results: By 1 December 2021, 46.8% HCWs of ASUGI had received the booster, 37.2% were immunized only with two doses of COVID-19 vaccines, 6.0% only with one dose and 10.0% were unvaccinated. During 1 March 2020–31 May 2022, 3571 primary against 406 SARS-CoV-2 recurrent infections were counted among HCWs of ASUGI, 59.7% (=2130/3571) versus 95.1% (=386/406) of which occurring from 1 December 2021 through 31 May 2022, respectively. All HCWs infected by SARS-CoV-2 during 1 December 2021 through 31 May 2022 presented mild flu-like disease. Compared to staff working in administrative services, the risk of primary as well as recurrent SARS-CoV-2 infection increased in HCWs with patient-facing clinical tasks (especially nurses and other categories of HCWs) and in all clinical wards but COVID-19 units and community health services. Regardless of the number of swab tests performed during the study period, primary infections were less likely in HCWs immunized with one dose of COVID-19 vaccine. By contrast, the risk of SARS-CoV-2 re-infection was significantly lower in HCWs immunized with three doses (aHR = 0.58; 95%CI: 0.41; 0.80). During the study period, vaccine effectiveness (VE = 1-aHR) of the booster dose declined to 42% against re-infections, vanishing against primary SARS-CoV-2 infections. Conclusions: Though generally mild, SARS-CoV-2 infections and re-infections surged during the Omicron transmission period. Compared to unvaccinated colleagues, the risk of primary SARS-CoV-2 infection was significantly lower in HCWs immunized just with one dose of COVID-19 vaccines. By Italian law, HCWs immunized only with one dose were either suspended or re-assigned to job tasks not entailing patient facing contact; hence, while sharing the same biological risk of unvaccinated colleagues, they arguably had a higher level of protection against COVID-19 infection. By contrast, SARS-CoV-2 re-infections were less likely in HCWs vaccinated with three doses, suggesting that hybrid humoral immunity by vaccination combined with natural infection provided a higher level of protection than vaccination only. In this stage of the pandemic, where SARS-CoV-2 is more infectious yet much less pathogenic, health protection measures in healthcare premises at higher biological risk seem the rational approach to control the transmission of the virus.

Background: The persistence of antibody levels after COVID-19 vaccination has public health relevance. We analyzed the determinants of quantitative serology at 9 months after vaccination in a multicenter cohort. Methods: We analyzed data on anti-SARS-CoV-2 spike antibody levels at 9 months from the first dose of vaccinated HCW from eight centers in Italy, Germany, Spain, Romania and Slovakia. Serological levels were log-transformed to account for the skewness of the distribution and normalized by dividing them by center-specific standard errors. We fitted center-specific multivariate regression models to estimate the cohort-specific relative risks (RR) of an increase of one standard deviation of log antibody level and the corresponding 95% confidence interval (CI), and combined them in random-effects meta-analyses. Finally, we conducted a trend analysis of 1 to 7 months’ serology within one cohort. Results: We included 20,216 HCW with up to two vaccine doses and showed that high antibody levels were associated with female sex (p = 0.01), age (RR = 0.87, 95% CI = 0.86–0.88 per 10-year increase), 10-day increase in time since last vaccine (RR = 0.97, 95% CI 0.97–0.98), previous infection (3.03, 95% CI = 2.92–3.13), two vaccine doses (RR = 1.22, 95% CI = 1.09–1.36), use of Spikevax (OR = 1.51, 95% CI = 1.39–1.64), Vaxzevria (OR = 0.57, 95% CI = 0.44–0.73) or heterologous vaccination (OR = 1.33, 95% CI = 1.12–1.57), compared to Comirnaty. The trend in the Bologna cohort, based on 3979 measurements, showed a decrease in mean standardized antibody level from 8.17 to 7.06 (1–7 months, p for trend 0.005). Conclusions: Our findings corroborate current knowledge on the determinants of COVID-19 vaccine-induced immunity and declining trend with time.

Background: Healthcare workers (HCWs) are particularly exposed to biological risk, including SARS-CoV-2 infection. In order to contrast the current pandemic and alleviate the burden of the disease on the healthcare system, a mass vaccination campaign against COVID-19 has been launched worldwide. Aim To evaluate the impact of COVID-19 vaccination in HCWs exposed to SARS-CoV-2, to describe the clinical presentation of COVID-19 in infected HCWs, and to investigate clinical and occupational risk factors for breakthrough infection. Design: Retrospective cohort study. Methods: The cohort of HCWs of Trieste Hospitals were followed up from 1 March 2020, to 30 November 2021 (21 months). All HCWs were periodically screened for SARS-CoV-2 infection by real-time PCR (RT–PCR) analysis. Clinical data were obtained through routine medical surveillance records. Risk factors for SARS-CoV-2 infection were investigated by univariable as well as multivariable logistic regression analysis. Results: Among 4394 HCWs routinely screened for SARS-CoV-2 by PCR on nasopharyngeal swab, a total of 800 incident cases were identified during the entire study period (1 March 2020 to 30 November 2021). Five hundred and sixty-four cases occurred before, and 236 after the start of the vaccination campaign against COVID-19, of whom 155 received a complete vaccination scheme before SARS-CoV-2 infection. Breakthrough infection was featured by mild or no symptoms and was significantly associated with the male sex, BMI > 25, and diabetes mellitus. Some categories of HCWs (physicians and nurse aids/auxiliary personnel) were at a higher risk of breakthrough infection. Conclusions: Fully vaccinated HCWs were less likely to acquire symptomatic as well as asymptomatic SARS-CoV-2 infection. Risk factors for SARS-CoV-2 infection after a full COVID-19 vaccination scheme included the male gender, diabetes mellitus, and overweight. HCWs with higher exposure to COVID-19 patients were at higher risk of breakthrough infection.

Vaccination coverage against COVID-19 among health care workers (HCWs) of the University Health Agency Giuliano-Isontina (ASUGI) of Trieste (North-eastern Italy) by 1 January 2022 was 90.4% with at least one vaccine dose, 84.9% with at least 2 doses, and 75.1% with 3 doses, 98.2% with Comirnaty (Pfizer BioNtech, New York, NY, USA) versus 1.8% with Spikevax (Moderna, Cambridge, MA, USA). From 1 October 2020 to 7 February 2022, 1652 SARS-CoV-2 infections were notified in HCWs of ASUGI Trieste. Although the overall risk of SARS-CoV-2 contagion increased over time, the rate of occupational infections progressively declined, from 42.5% during the second COVID-19 wave to 15.6% in the fifth. Between 1 January–7 February 2022 (a period dominated by the Omicron variant), albeit no COVID-19-associated hospitalizations were recorded in HCWs of ASUGI Trieste, 669 SARS-CoV-2 infections were counted against 367 cases observed from 1 October to 31 December 2020, the 3 months preceding the implementation of the vaccination campaign against COVID-19. Job tasks and health care settings turned out to be the most significant risk factors for SARS-CoV-2 infection. However, the effect of workplace prevailed over job task on the biological risk, with greater rates of SARS-CoV-2 infections observed among HCWs operating in areas with higher levels of circulation of the virus, particularly COVID-19 dedicated units.