SARS-CoV-2 Seroprevalence Study in Pimpri-Chinchwad, Maharashtra, India Coinciding with Falling Trend – Do the Results Suggest Imminent Herd Immunity?

Background: COVID-19 is a public health concern currently demanding continuous efforts to understand its epidemiology. Pimpri-Chinchwad township with a population of over 25 lakhs is located in Maharashtra, one of the worst affected states in India. After incidence peaked in the township in mid-September 2020, cases started declining even as lockdown restrictions were eased. This was somewhat unusual as in other countries cases spiked on easing restrictions. At this juncture, a seroprevalence study was conducted to understand the transmission dynamics of the pandemic in this region. Methods: We carried out a population based seroprevalence study for IgG antibodies for SARS-CoV-2 among 5000 residents over 11 years selected by cluster random sampling. We selected 50 clusters in slums, 80 clusters in tenements and 70 clusters from housing societies. The field work for collection of samples was carried out from 07 October to 17 October 2020. We used kit from Abbott (SARS-CoV-2 IgG), which employs chemiluminescent microparticle immunoassay (CMIA) technology. Statistical analysis: Prevalence of IgG antibodies was expressed as point estimates with 95% confidence intervals. These were weighted for areas and cluster effect and further adjusted for test performance. Results: The overall seropositivity for IgG antibodies was 34.04% (95% confidence interval 31.3% to 36.8%). Slum dwellers had positivity rate of 40.9% (95% confidence interval 37.0% to 44.7%), those in tenements had positivity of 41.2% (95% confidence interval 37.7% to 44.8%) and more affluent people living in housing societies had positivity of 29.8% (95% confidence interval 25.8% to 33.8%). Conclusion: The study indicates that a considerable proportion of the population had encountered the novel coronavirus approaching partial, if not complete, herd immunity, which may partly explain the declining trend in spite of easing of lockdown restrictions.