New Delhi: India had an estimated 6.4 million coronavirus cases in early May, according to a survey conducted by the Indian Council of Medical Research (ICMR), while the official count was 85,940 on May 15.

The ICMR’s serological survey suggests that the country may have missed a significant proportion of cases in the first three months of the pandemic. The number of cases in the country was over 4.5 million as of Friday morning.

“Based on the overall adjusted seroprevalence of 0.73% and reported number of Covid-19 cases, it was estimated that for every RT-PCR (reverse transcription polymerase chain reaction) confirmed case of Covid-19, there were 82-130 infections in India,” the ICMR said in a report 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 nationwide survey. The seroprevalence ranged between 0.62% and 1.03% across districts.

“Population-weighted seroprevalence after adjusting for test performance was 0.73%. Males living in urban slums and occupation with high risk of exposure to potentially infected persons were associated with seropositivity. A cumulative 6,468,388 adult infections were estimated in India by early May,” ICMR said. A sero survey tests for antibodies that are triggered by a virus and can indicate the percentage of people who may have been exposed.

According to Girdhar Babu, a member of the ICMR taskforce, the findings had both bad and good news. “In early May 2020, it indicates that most of the population remains susceptible to infection. Seroprevalence estimates conducted later in the epidemic or in the settings with higher prevalence, will provide more reliable infection-to-case and infection-to-fatality ratios,” he said.

According to the survey “the overall ICR (infection to case ratio) was between 81.6 and 130.1 with May 11 and May 3, 2020 as plausible reference points for reported cases. The IFR (infection to 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 as plausible reference points for reported deaths,” it said.

Significantly, 856,062 infections were in zero-case districts. At the time of the survey, there were 233 zero-case districts. The report found India’s infection fatality rate (IFR) was underestimated. “Considering that the death reporting in India is incomplete, and differences in access to testing facilities across districts necessary for declaring the Covid-19 confirmed deaths, the present IFR is likely an underestimate,” it said.

The study found that while the overall IFR based on the sero survey findings was much lower than that reported from Santa Clara County in the US (0.12-0.2%), Iran (0.08-0.12%), Brazil and Spain (1%), the IFR from the high-stratum districts, where reporting is assumed to be more complete, was similar to them.

Contrary to previous understanding, the virus was already widespread in rural areas, with 69.4% of the population surveyed showing IgG antibodies. The figure was 15.9% in urban slums and 14.6% in other urban areas.