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

Introduction This study was aimed at assessing the vaccine effectiveness (VE) of Covishield, which is identical to the AstraZeneca vaccine, in preventing laboratory-confirmed Covid-19. Methods Using a 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 matched controls, i.e., positive and negative for SARS-CoV-2 by RT-PCR, was obtained. The cases and controls were matched for age (±3 years) and date of testing (±3 days). The groups were compared using multivariable conditional logistic regression to calculate odds ratios (OR), with adjustment for gender, occupational role, presence of symptoms and presence of a comorbidity condition. Per cent vaccine effectiveness (VE) was calculated as 100 x (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. In view of a difference in the proportion of cases and controls who had symptoms, a separate analysis of data from 203 pairs where both the case and the control had symptoms was done, which showed VE of 58% (28%-75%) and 64% (38%-78%) after one dose and two doses, 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 the protection after two doses being slightly higher than after one dose, and a particularly high protection rate against severe forms of disease.