NEW DELHI: A total of 64,68,388 adult Covid-19 infections were estimated in India by early May, even though the adjusted all-India seroprevalence was a low 0.73%, according to the results of a serological survey conducted by the Indian Council of Medical Research (ICMR) released on Friday.
While the survey indicated that less than 1% of the adult population was exposed to the novel coronavirus by mid-May 2020, the official tally at the time was even lower at only 85,940 cases detected till May 15. This would indicate most cases may have been missed during the first few months of the outbreak in the country.
This could have been because the testing strategy then was tailored by limited test kits and lab capacity and also because a significant number of infections may have been asymptomatic. A sero survey estimates the percentage of population exposed to a virus by finding the presence of antibodies. Antibodies indicate that a person had been infected but is no longer so.

Though the ICMR had shared some findings of the study in June, the final study was published in the Indian Journal of Medical Research on Friday. A total of 30,283 households were visited and 28,000 individuals enrolled for the sero survey across the country. In June, too, ICMR had estimated prevalence of infection at 0.73%. It had also said risk of the spread was 1.09 times higher in urban areas as compared to rural areas. Whereas in urban slums, it was 1.89 times higher than rural areas.
“Males, living in urban slums and occupation with high risk of exposure to potentially infected persons were associated with seropositivity,” the study said.
Of the 157 people found positive in the survey, 43.3% were in the age group 18-45, 39.5% were those aged 46-60 and the remaining 17.2% were over the age of 60. Seropositivity was, however, highest in the 46-60 age group, with 62 out of 9,525, or 0.65%, testing positive and lowest in the youngest age group at 0.5% (68 out of 13,552).
“The IFR (infection fatality ratio) in the surveyed districts from high stratum, where death reporting was more robust, was 11.72 to 15.04 per 10,000 adults, using May 24 and June 1, 2020 as plausible reference points for reported deaths,” the study said.
The survey divided districts into four strata and, interestingly, 8,56,062 infections were in zero-case districts. At the time of the survey, there were 233 zero-case districts in India.