Effectiveness of the ChAdOx1 nCoV-19 Vaccine against Laboratory-confirmed Cases of COVID-19: A Test-negative Case-control Study from Central Kerala, India

Abstract

Introduction: The COVID-19 pandemic continues to impact livelihoods worldwide, and in the absence of specific antivirals, the vaccine remains the main weapon against it. Assessing the effectiveness of vaccines against Coronovirus Disease 2019 (COVID-19) in practice is crucial as COVID-19 variants continue to emerge, and public health decisions must be supported by scientific risk-benefit considerations. Aim: To determine the Vaccine Effectiveness (VE) of two doses of the ChAdOx1 nCoV-19 (Covishield) vaccine in preventing laboratory-confirmed COVID-19. Materials and Methods: A test negative case control design was used to determine the VE in total of 702 individuals which included 351 laboratory confirmed cases using Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) and 351 controls who tested negative among those who attended the testing sites of the Urban Family Health Centre of Government Medical College Kottayam, Kerala, India from July 2021 to September 2021. Details regarding vaccination status, sociodemographic factors, symptoms, and co-morbidities were collected from consented and eligible participants. The collected information was entered into a proforma, which was later entered into MS Excel and analysed using R software version 4.1.3. The groups were compared using binary logistic regression to calculate the adjusted Odds Ratio (aOR) with adjustment for gender, age group, education, occupation, presence of symptoms, and comorbidity status. VE% was calculated as 100 * (1-aOR). Results: The median age (interquartile range) of cases and control was 44 (33-57) years and 50 (35-60) years, respectively. The VE of two doses of the ChAdOx1 nCoV-19 vaccine in protecting against laboratory-confirmed COVID-19 was 87% (95% CI 78-92), with an aOR of 0.13. A separate analysis was conducted to determine the VE among symptomatic individuals, which showed a VE of 89% (95% CI 79-94), with an aOR of 0.11. Conclusion: Two doses of the ChAdOx1 nCoV-19 vaccine are protective against laboratory-confirmed cases of COVID-19.