Effectiveness of Covishield vaccine in preventing Covid-19 – A test-negative case-control study

INTRODUCTION: This study aimed at assessing the vaccine effectiveness (VE) of Covishield, which is identical to AstraZeneca vaccine, in preventing laboratory-confirmed Covid-19. METHODS: Using test-negative case-control design, information on vaccination status of cases with Covid-19 among healthcare workers in our institution in Puducherry, India, and an equal number of controls matched for age and date of testing, was obtained. The groups were compared using multivariable conditional logistic regression to calculate odds ratios (OR). VE was calculated as 100*(1–adjusted odds ratio)%. RESULTS: Using data from 360 case-control pairs, VE of one dose and of two doses, in providing protection against Covid-19 was 49% (95% CI: 17%-68%) and 54% (27%-71%), respectively. Among cases with moderately severe disease that required oxygen therapy, VE following any number of vaccine doses was 95% (44%-100%). CONCLUSION: Covishield vaccine protected significantly against Covid-19, with a higher protection rate against severe forms of disease.