Covishield (TM) (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 (TM) (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 13 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."