METHOD: We conducted a prospective cohort study of 4252 healthcare workers aged 18 and above, who tested negative for the SARS-CoV-2, and partially or fully vaccinated with the ChAdOx1nCoV-19 vaccine at baseline. After completing the baseline questionnaire, healthcare workers were followed up for one year.

RESULTS: 2283 out of the 4252 (53.7%) healthcare workers had post-vaccination infections, with an overall incidence of 95.7 cases per 100 person-years (95% CI: 91.8-99.7) of follow-up. The incidence of breakthrough infection was 82.0 cases per 100 person-years (95% Cl 78.0-86.0). In a multivariable Cox regression, age, vaccination status, occupation, clinical stations, frontline status and previous SARS-CoV-2 infections were significantly associated with post-vaccination infections. Compared to non-clinical healthcare workers, nurses (HR = 1.91, 95% CI: 1.69-2.17) and doctors (HR = 1.37, 95% CI: 1.24-1.73) had a higher risk of post-vaccination infections. Similarly, elderly individuals (HR = 1.04, 95% CI: 1.02-1.05) and those with comorbidities (HR = 1.86, 95% CI: 1.67-1.73) were more likely to develop post-vaccination infections. Frontline healthcare workers and healthcare workers stationed at the point-of-entry services (emergency and outpatient clinics) had a high rate of infections. However, previous SARS-CoV-2 infections (HR = 0.80, 95% CI: 0.71-0.53) and full vaccination (HR = 0.56, 95% CI: 0.51-0.62) conferred some protection, despite an overall rise in infection post vaccination incidence.