Background and Objective:
In September 2022, the bivalent Comirnaty Original/Omicron BA.4-5 vaccine was authorized for use as a booster dose for individuals aged 12 years and older. The aim of this retrospective study was to assess the acceptance of the fourth dose among healthcare workers (HCWs) of an Italian teaching hospital and to describe changes in uptake trend compared with the previous COVID-19 vaccination campaigns, considering that the fourth dose is not mandatory.

Methods:
Pearson’s X2 test was used to test the correlation between the fourth dose vaccination rate and the previous doses vaccination rate. A linear regression model was adopted to predict fourth dose vaccination coverage. Multivariate logistic regression was performed to investigate the influence of sex, age, and occupation on vaccination uptake.

Results:
Overall, 8159 employees were included in the study. Among them, 1077 received the bivalent booster dose between September and December 2022. Compared with men, women had a lower tendency to receive vaccination, but this difference was not statistically significant. The mean age at the time of vaccination was 44.1 years, and the probability of vaccination increased significantly (p<0.001) as age increased. The logistic regression showed that being a physician is significantly (p<0.001) associated with higher uptake compared to nurses (OR 0.25, 95%CI 0.20-0.31), other HCWs (OR 0.21, 95%CI 0.16-0.27) and administrative employees (OR 0.33, 95%CI 0.25-0.42). In contrast, working in low-risk, medium-risk or high-risk operating units did not affect vaccine uptake significantly. When analyzing the uptake of previous COVID-19 vaccine doses, the expected value, according to the linear regression model, was estimated to be 30.7%, but the observed value was significantly lower (13.0%).

Conclusion:
To mitigate the socio-economic and health effects of COVID-19, it is critical to further promote vaccination of HCWs, given their crucial role in vulnerable settings.