Covishield™ (ChAdOx1) vaccine effectiveness and epidemiological risk factors of COVID-19 infection among frontline workers during second wave of COVID-19 pandemic, New Delhi: A case–control study

Abstract

Background: Frontline workers were the first cohorts vaccinated with Covishield™ (ChAdOx1 nCoV-19) vaccine with dose-interval of 4–6 weeks. We evaluated vaccine effectiveness (VE) of Covishield and studied epidemiological risk factors associated with COVID-19 during second wave of COVID-19 pandemic. Methods: We conducted a 1:3 case-control community-based study, as per WHO protocol. We identified case-patients from COVID-19 surveillance system and recruited controls from the same community as per the WHO protocol. Information was obtained through questionnaire; and all potential confounders were identified to evaluate VE. Results: We enrolled 243 case-patients and 712 controls. Adjusted VE of fully vaccinated was 74% (95% confidence interval [CI]: 53%–86%) against infection and 91% (95% CI: 78%–97%) against moderately severe disease. Pre-infection high-risk exposure events such as ccontact with COVID-19-positive patient, visit to a crowded place, and attending social-gathering in confined space were significantly associated with contracting infection, with odds ratios 10.1 (95% CI: 5.6–18.3), 6.0 (95% CI: 1.8–20.2) and 3.9 (95% CI: 1.4–10.5) respectively. The use of double-mask and past COVID-19 infection was 60% and 70% protective, respectively. Conclusion: Covishield vaccine is highly effective against infection and mainly against disease-severity during high-transmission settings. We recommend three-layer shield to minimize breakthrough and re-infections comprising of vaccination, double-masking, and avoiding “pre-infection high-risk exposure events.”