ABSTRACT
OBJECTIVES: We aimed to investigate real-world vaccine effectiveness (VE) for Oxford-AstraZeneca (ChAdOx1) and CoronaVac against laboratory-confirmed COVID-19 infection among healthcare workers (HCWs). METHODS: We conducted a retrospective cohort study among HCWs (aged ≥18 years) working in a private healthcare system in Brazil between January 1, 2021 and August 3, 2021. To assess VE, we calculated VE=1-RR (rate ratio), with RR determined by adjusting Poisson models with the occurrence of COVID-19 infection as the outcome, and the vaccination status as the main exploratory variable. We used the logarithmic link function and simple models adjusting for sex, age and job types. RESULTS: 13,813 HCWs met the inclusion criteria for this analysis. 6,385 (46.2%) received the CoronaVacvaccine, 5,916 (42.8%) received the ChAdOx1 vaccine, and 1,512 (11.0%) were not vaccinated. Overall, COVID-19 infection cases happened in 6% of unvaccinated HCWs, 3% of HCWs receiving two doses of CoronaVacvaccine, and 0.7% of HCWs receiving two doses of ChAdOx1 vaccine (p-value< 0.001). In the adjusted analyses, the estimated VE was 51.3% for CoronaVac, and 88.1% for ChAdOx1 vaccine. Both vaccines reduced the number of hospitalizations, the length of hospital stay, and the need of mechanical ventilation. Nineteen SARSCoV-2 samples from nineteen HCWs were screened for mutations of interest. Eighteen out of nineteen of those samples were Gamma SARS-CoV-2 variant. CONCLUSIONS: While both COVID-19 vaccines (viral vector and inactivated virus) can significantly prevent COVID-19 infection among HCWs, CoronaVac was much less effective. The COVID-19 vaccines were also effective even against a dominant Gamma variant.