Protective Effect Conferred by Prior Infection and Vaccination on COVID-19 in a Healthcare Worker Cohort in South India

Abstract

Background: The emergence of newer variants with the immune escape potential raises concerns about breakthroughs and re-infections resulting in future waves of infection. We examined the infection rates among a cohort of healthcare workers (HCW) in South India during the second wave driven mainly by the delta variant and examined the protective effect of prior COVID-19 disease and vaccination.\xa0\xa0 \n \nMethods: Symptomatic HCWs were routinely tested by RT-PCR per institutional policy. Vaccination was offered to all HCWs between January and June 2021, and the details were documented. We set up a non-concurrent cohort to estimate infection rates and estimated efficacy of prior infection and vaccination between 16th Apr to 30th May, 2021, using a cox-proportional hazards model with time-varying covariates adjusting for daily incidence.\xa0 \n \nFindings: Between June 2020 and May 2021, 2735 (23.9%) of 11,405 HCWs were infected, with 1412, including 32 re-infections, reported during the second wave. 6863 HCWs received two doses of vaccine and 1905 one dose. The protective efficacy of prior infection against symptomatic infection was 86.0% (95%CI 76.7% - 91.6%). Vaccination combined with prior infection provided 91.1% (95%CI 84.1% - 94.9%) efficacy. In the absence of prior infection, vaccine efficacy against symptomatic infection during the second wave was 31.8% (95%CI 23.5% – 39.1%).\xa0 \n \nInterpretation: Prior infection provided substantial protection against symptomatic re-infection and severe disease during a delta variant driven second wave. Deferring vaccination for previously symptomatically infected individuals is a pragmatic strategy to overcome vaccine supply constraints. However, the duration of protection requires further evaluation. \n \nFunding None to declare. \n \nDeclaration of Interest: None to declare. \n \nEthical Approval: This study was approved by the institutional ethics committee and research \nboard (IRB Min no. 13980).