Effectiveness of COVID-19 vaccine (Covaxin) against breakthrough SARS-CoV-2 infection in India.

India approved COVID-19 vaccine called Covaxin, developed by the Indian Council of Medical Research and Bharat Biotech Ltd. The primary objective of the study was to estimate the effectiveness of Covaxin in preventing breakthrough SARS-CoV-2 infection in healthcare workers (HCWs). A test-negative matched case-control study was conducted among HCWs of tertiary care hospital in Eastern India. Any HCW who tested positive for COVID-19 using RT-PCR during April and May 2021 was taken as the case. The HCWs who tested negative for COVID-19 by RT-PCR were considered as controls after matching with the date of testing and profession of the cases. Vaccination data were collected from the institution's vaccine database and recall. In case of discrepancy, it was confirmed from the CoWIN portal (cowin.gov.in). The sample size was 670 participants (335 pairs). Conditional logistic regression models were used to calculate the adjusted odds ratio for breakthrough SARS-CoV-2 infection. Vaccine effectiveness was calculated using the following formula: VE = (1-aOR) × 100%. Sensitivity analysis was done for effectiveness of Covaxin, excluding Covishield vaccination. The mean age of participants was 29.1 years (SD = 7.1), and the majority were males (55.2%). Among the study participants, 60% were completely vaccinated, 18.51% were partially vaccinated, and 21.49% were unvaccinated. After adjusting for age, gender, type of household and past history of COVID-19 disease in conditional logistic models, the vaccine effectiveness was 22% (aOR 0.78, 95% CI: 0.52-1.17; p = .233). Sensitivity analysis with Covaxin showed an effectiveness of 29% (aOR 0.71, 95% CI: 0.47-1.08; p = .114) for preventing breakthrough SARS-CoV-2 infection.