Second wave of the Covid-19 pandemic in Delhi, India: high seroprevalence not a deterrent?

Sharma, N.; Sharma, P.; Basu, S.; Bakshi, R.; Gupta, E.; Agarwal, R.; Dushyant, K.; Mundeja, N.; Marak, Z.; Singh, S.; Singh, G.; Rustagi, R.; Sarin, S. K.

Background We 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. Methods Individuals of age more than or equal to 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). Results Antibody 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 more than or equal to 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. Conclusions The presence of infection induced immunity from SARS-CoV-2 even in more than one in two people can be ineffective in protecting the population. Keywords: Covid-19; Serosurvey; Delhi; Pandemic wave