Introduction The study objective was to determine the breakthrough infection rate of Covid-19 (SARS-CoV-2) infection in those vaccinated with either BBV152 or AZD1222 (ChAdOx1-S) vaccine among healthcare workers (HCWs). Methods A cross-sectional analysis was conducted a medical college and hospital complex in Delhi, India through telephonic interviews among HCWs who had received at-least one dose of a Covid-19 vaccine during January to March 2021. Breakthrough infections were operationally defined as occurrence of Covid-19 infection 14 days after administration of two doses of either Covid-19 vaccine. Results We enrolled 326 HCWs with mean (SD) age of 29.1 (85.9) years including 212 (65%) males. Two hundred eighty (90.9%) HCWs were fully vaccinated while 46 (14.1%) were partially vaccinated. There were 168 (51.5%) BBV152 and 158 (48.5%) AZD1222 (ChAdOx1-S) recipients. A total of 36 (11%, 95% C.I. 8.1, 14.9) breakthrough infections were reported in the HCWs. The median (IQR) time until incidence of Covid-19 infection since receiving second dose of either Covid-19 vaccine was 46 (28.2, 54.7) days. Furthermore, Covid-19 infections occurred in 65 (19.9%, 95% C.I 15.9, 24.6) HCWs vaccinated with at-least one dose of vaccine but prior to receiving their second dose or <14 days post second dose. Most breakthrough infection cases (94.4%) were mild and did not require supplemental oxygen therapy. HCWs with past history of natural Covid-19 infection with recovery were 4.5 times less likely to experience a Covid-19 infection after partial vaccination. Conclusion Nearly one in nine HCWs experienced a Covid-19 breakthrough infection in the present study.

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