ABSTRACT
Background: This study was carried out to evaluate the effectiveness of partial and full vaccination with ChAdOx1 nCoV-19 (COVISHIELD) to prevent the development of moderate or severe illness among COVID-positive cases. Methods: This prospective cohort study was conducted among Armed Forces personnel deployed in Northern India who were found COVID positive during the study period between January and June 2021. Information about the vaccination status, age and comorbidities was collected at the time of diagnosis. Classification of COVID cases as moderate or severe was performed as per criteria given by the Government of India. Individuals were considered partially vaccinated three weeks after one dose and fully vaccinated two weeks after the second dose. Risk ratio and vaccine effectiveness (VE) to prevent moderate or severe disease among COVID cases were calculated. Results: A total of 2005 COVID-19 patients were included in our study. Partial vaccination and full vaccination with ChAdOx1 nCoV-19 offered 13% (95% credible interval (CI): -56.8%, 52.8%) and 66.6% (95% CI: 34.9%, 84.6%) protection against progression to moderate/severe illness among COVID-positive individuals. The risk of moderate-severe disease among COVID-positive cases occurring 4-11 weeks after the first dose was also lesser among those who had taken the second dose of vaccine than individuals who have been vaccinated with only one dose. Conclusion: Interval between the first and second doses of ChAdOx1 nCoV-19 vaccine should be reduced to 4-6 weeks, as partial vaccination offers lower protection against the development of moderate-severe illness after COVID infection.