Search

Background: Repeated cross-sectional serosurveys in the same geographic area establish the trend of the evolving pandemic. We present the findings of the third round of a national serosurvey to estimate the seroprevalence of SARS-CoV-2 infection among the general population and health care workers of India. Methods: We conducted the third population-based serosurvey between Dec 18, 2020 and Jan 6, 2021 in the same 700 villages or wards from 70 districts in 21 states across India, which were selected for the first and second serosurveys. We enrolled from each district, at least 400 individuals aged ≥ 10 years from general population and 100 HCWs from sub-district level public health facilities. Serum samples from general population were tested for the presence of IgG antibodies against nucleocapsid (N) and spike protein (S1-RBD) of SARS-CoV-2 using the Abbott and Siemens assays respectively, whereas sera from HCWs were tested for anti-S1-RBD. For general population, sera positive for either of the antibodies were considered positive, while sera positive for anti-S1-RBD were considered as positive for HCW. Weighted seroprevalence estimates were adjusted for sensitivity and specificity of respective assays. Findings: Of the 28,598 sera from general population, 4585 (16%) had IgG antibodies against N, 6647 (23.2%) against S1-RBD and 7436 (26%) against either. The weighted and assay characteristic adjusted seroprevalence against either of the antibodies was 24.1 (95%CI: 23.0% to 25.3%). Seroprevalence was lower in rural areas (21.4%, 95% CI: 20.3% to 22.6%) compared to urban non-slum (29.4%, 95% CI: 26.9% - 32.1%) and slum areas (34.6%, 95% CI: 31.0% to 38.3%). Among 7385 HCWs, the seroprevalence of anti-S1-RBD IgG antibodies was 25.6% (95% CI: 23.5% to 27.8%). Interpretation: Nearly one in four individuals aged 10 years or older from general population as well as HCWs were exposed to SARS-CoV-2 by December 2020 amounting to 271 million infections in India. Funding Statement: Indian Council of Medical Research Declaration of Interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf and declare no competing interests Ethics Approval Statement: The project was approved by Institutional Human Ethics Committee at ICMR-National Institute of Epidemiology.

Background: The first round of national serosurvey in India was conducted in May-June 2020 among adults from 21 States. The second serosurvey was conducted in August-September 2020 to estimate the nationwide seroprevalence for SARS-CoV-2 infection in the general population aged ten years and above. Methods: The household serosurvey was conducted among individuals aged ten years and above in the same 700 villages and wards from 70 districts selected during the first serosurvey. Blood samples were tested using the Abbott SARS-CoV-2 IgG assay. Seroprevalence was estimated after applying the sampling weights and adjusting for clustering and assay characteristics. In order to compare the adult seroprevalence between the two surveys, we randomly selected one adult serum sample from each household. Findings: The weighted and adjusted prevalence of infection among 29,082 individuals aged ten years and above was 6·6% (95% CI: 5·8% - 7·4%). The seroprevalence among adults was 7·1% (95% CI: 6·2% – 8·2%). Seroprevalence was similar across age groups, sex, and occupation. Seroprevalence was highest in urban slum areas followed by urban non-slum and rural areas. We estimated a cumulative 74·3 million infections in the country, with 26 – 32 infections for every reported COVID-19 case by August 2020. Interpretation: Nearly one in 15 individuals aged ten years and above had SARS-CoV-2 infection by August 2020. The adult seroprevalence increased ten times between May and August 2020. Lower infection to case ratio in August compared to May reflects a substantial increase in testing across the country. Funding: The study was funded by the Indian Council of Medical Research, New Delhi, India. The study sponsor was involved in reviewing the study design, writing of the manuscript and the decision to submit the paper for publication. Declaration of Interests: The authors declare no competing interests. Ethics Approval Statement: The authors obtained written informed consent from individuals aged 18 years and older. We obtained assent from children aged between 10–17 years, and written informed consent from their parents or guardians prior to the survey. The Central Ethics Committee of Health Research of Indian Council of Medical Research and the Institutional Human Ethics Committee of ICMR-National Institute of Epidemiology, Chennai approved the study protocol.