The findings of the first national population-based serosurvey indicate that 64 lakh adults could have been exposed to SARS-CoV-2 infection by early May 2020. As percentage of the total population, it means 0.73 per cent of adults in India were exposed to the virus by that time. The seroprevalence ranged between 0.62 and 1.03 per cent across the four strata of districts.
The findings of the survey, published in the Indian Journal of Medical Research, suggest further national and local serosurveys to frame public health strategies for containment and mitigation of the epidemic in various parts of the country. Conducted by ICMR institutions, the randomly sampled, community-based survey was conducted during May 11 to June 4, in 700 villages, selected from the 70 districts of the 21 states of India, categorised into four strata based on the incidence of reported COVID-19 cases.
Four hundred adults per district were enrolled from 10 clusters with one adult per household. Serum samples were tested for IgG antibodies using COVID Kavach ELISA kit. All positive serum samples were re-tested using Euroimmun SARS-CoV-2 ELISA. Adjusting for survey design and serial test performance, weighted seroprevalence, number of infections, infection to case ratio (ICR) and infection fatality ratio (IFR) were calculated.
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The findings indicate that males, living in urban slums and occupation with high risk of exposure to potentially infected persons were associated with seropositivity. The response rate in different strata ranged from 86.9 to 95.9 per cent. Nearly half of the 28,000 survey participants were aged between 18 and 45 years and 51.5 per cent were female. In all, 18.7 per cent of the participants had an occupation with a high risk of exposure to potentially infected persons.
A cumulative 6,468,388 adult infections were estimated in India by the early May. The analysis also shows that among the respondents, 486 individuals (1.7 percent) reported a history of respiratory symptoms in the preceding one month, of whom, 44.7 per cent sought medical care and 30.9 per cent of those who sought care were hospitalised. Meanwhile, 151 (0.5 percent) individuals reported a history of contact with a COVID-19 case and 70 (0.3 percent) reported that they were tested for COVID-19 before the survey.
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The findings indicate that the overall seroprevalence in India was low, with less than one per cent of the adult population exposed to SARS-CoV-2 by mid May 2020. "The low prevalence observed in most districts indicates that India is in early phase of the epidemic and the majority of the Indian population is still susceptible to SARS-CoV-2 infection. It is, therefore, necessary to continue to implement the context-specific containment measures including the testing of all symptomatics, isolating positive cases and tracing high risk contacts to slow transmission and to prevent the overburdening of the health system," it points out.
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