RESUMEN

BackgroundWe report the findings of a large follow-up community-based serosurvey and correlating it with the COVID-19 test-positivity rate and the case load observed during the peak of the second wave of the Covid-19 pandemic in Delhi, India. MethodsIndividuals of age [≥]5 years were recruited from 274 wards of the state (population [~] 19.6 million) during January 11 to January 22 2021. A total of 100 participants each were included from all the wards for a net sample size of [~]28,000. A multi-stage sampling technique was applied for selection of participants for the household serosurvey. Anti SARS CoV-2 IgG antibodies were detected by using the VITROS assay (90% Sn, 100% Sp). ResultsAntibody positivity was observed in 14,298 (50.76%) of the 28,169 samples. The age, sex and district population weighted seroprevalence of the IgG SARS-CoV-2 was 50.52% (95% C.I. 49.94-51.10) and after adjustment for assay characteristics was 56.13% (95% C.I. 55.49-56.77). On adjusted analysis, participants aged [≥]50 years, of female gender, housewives, having ever lived in containment zones, urban slum dwellers, and diabetes or hypertensive patients had significantly higher odds of SARS-CoV-2 antibody positivity. The peak infection rate and the test positivity rate since October 2020 were initially observed in mid-November 2020 with a subsequent steep declining trend, followed by a period of persistently low case burden lasting until the first week of March 2021. This was followed by a steady increase followed by an exponential surge in infections from April 2021 onwards culminating in the second wave of the pandemic. ConclusionsThe presence of infection induced immunity from SARS-CoV-2 even in more than one in two people can be ineffective in protecting the population.