COVID-19 in City Council Civil Servants, 1 March 2020-31 January 2023: Risk of Infection, Reinfection, Vaccine Effectiveness and the Impact of Heterologous Triple Vaccination

Background: The risk of COVID-19 increases in any occupation entailing intense social interactions. This study aimed to investigate the impact of COVID-19 among civil servants of Trieste city council (northeastern Italy) over the entire pandemic. Methods: The crude incidence rate of COVID-19 was estimated from 1 March 2020 to 31 January 2023 by explanatory factors, expressing the estimate as COVID-19 events x 10,000 person-days (P-d) at risk. A multivariable Cox proportional hazard regression model was fitted to examine the risk of primary COVID-19 infection and reinfections, reporting adjusted hazard ratios (aHR) with 95% confidence interval (95% CI). Results: The cohort of Trieste city council was mainly composed of administrative clerks (48.5%), nursery teachers (33%), technicians (9.9%) and local police officers (8.5%). Between 1 March 2020 and 31 January 2023, 1444 (62.4%) employees tested positive for SARS-CoV-2 at least once and 18.1% (=262/1444) at least twice. By the end of this study, 55% (N = 1272) of employees had received at least three doses of COVID-19 vaccine, whereas 19.7% (N = 457) remained unvaccinated. At multiple Cox regression analysis, the adjusted risk of primary COVID-19 events during the entire study period increased in employees aged 40-49 years (aHR = 1.65; 95% CI: 1.01; 2.71), females (aHR = 1.28; 95%CI: 1.12; 1.45), local police officers (aHR = 1.82; 95%CI: 1.50; 2.22) and nursery teachers (aHR = 1.27; 95%CI: 1.13; 1.43). However, whilst the risk of primary infections in police officers increased already during the Alpha transmission period (aHR = 6.82; 95%CI: 4.48; 10.40), progressively reducing across subsequent variants, for nursery teachers, it increased during the Delta wave (aHR = 2.42; 1.70; 3.44), reducing with Omicron (aHR = 1.23; 95%CI: 1.07; 1.40). Compared to unvaccinated colleagues, during the entire study period the risk of primary infections was significantly lower in employees immunized with three (aHR = 0.42; 95%CI: 0.36; 0.47) or four (aHR = 0.30; 95%CI: 0.23; 0.40) doses of COVID-19 vaccine, for a vaccine effectiveness (VE) of 58% and 70%, respectively. The protective effect of vaccination against primary infections was confirmed in the sub-group analysis by main pandemic waves, for a VE of 75% for one dose against 99% for two doses during the Alpha transmission period, slightly reducing to 59% and 70% in Delta time, respectively. During the Omicron wave, the risk of primary SARS-CoV-2 infections diminished significantly with three (aHR = 0.42; 95%CI: 0.36; 0.49) or four vaccine doses (aHR = 0.09; 95%CI: 0.05; 0.16), for a VE of 58% and 91%, respectively. Moreover, the risk of primary SARS-CoV-2 reinfections during the entire study period reduced with one (aHR = 0.47; 95%CI: 0.27; 0.82), two (aHR = 0.42; 95%CI: 0.30; 0.58), three (aHR = 0.32; 95%CI: 0.24; 0.44) or four vaccine doses (aHR = 0.14; 95%CI: 0.05; 0.46), for a VE of 53%, 58%, 68% and 86% against reinfections, respectively. No significant difference in VE was associated with heterologous versus homologous triple vaccination, both against primary infections or reinfections. Conclusions: Primary SARS-CoV-2 infections were more likely among nursery teachers and local police officers. The risk of both primary infections and reinfections reduced with higher number of doses of COVID-19 vaccine, regardless of the pandemic wave. Since city council civil servants were swab tested on demand or for contact tracing, the estimation of COVID-19 risk and VE largely missed aymptomatic SARS-CoV-2 infections. On the one hand, the present study confirmed the protective effect of COVID-19 vaccination against symptomatic SARS-CoV-2 infections; on the other hand, it highlighted not only the importance of continuous booster doses to keep up the humoral immunity over time but also the importance of updated vaccine formulations to prevent and control the spread of a highly mutable virus. Moreover, the protective effect of the first two doses against reinfections confirmed the efficacy of hybrid immunity during Omicron time.