Around 7% of India’s adult population may have been exposed to the coronavirus till the last fortnight of August, according to the second national sero-survey by the Indian Council of Medical Research (ICMR).
This is roughly a 10-fold jump in numbers from the first sero-survey conducted by the Council across 70 districts in 21 States that sought to estimate the likely number of infected until early May. The people tested in the second survey were drawn from the same villages and clusters as the first, in which the scientists said that 0.73% of adults — or about 6.4 million — across the country were likely infected.
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No figures were shared on the likely number of infections by ICMR Director General, Dr Balram Bhargava, in his presentation on the sero-survey on Tuesday. But 7% of the population works out to about 62 million persons.
Sero-surveys are conducted by drawing blood samples and checking for a specific class of antibodies called IgG that appear within two weeks of an infection. Because it is yet unclear how long antibodies to the coronavirus detectably persist in the body, their presence only indicates past exposure to — and not presence of — the virus.
During the first survey, it emerged that there were 82-130 infections for every confirmed COVID-19 positive case. That number has now dropped to 26-32 infections, which according to Dr. Bhargava, was the result of ramped up testing and early case detection.
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In September there were 29 million tests, compared to 23 million in August, 10.5 million in July and 30,000 in March according to Health Secretary Rajesh Bhushan.
However, the numbers also suggest that the country still has an overwhelming majority of its population yet to be exposed to the virus and therefore, is far from any peak’ or ‘herd immunity’ levels. These refer to approximate estimates that show what percentage of the population needs to be exposed to the virus so that the its spread is curtailed.
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Urban slums at risk
“The risk of being exposed to the virus is real and we have to continue with our measures. Risk in urban slums is twice that of non-slums and four times that in rural settings,” said Dr. Bhargava.
One in fifteen individuals above 10 were exposed to the virus by August and there was no difference by age group and gender.
“In light of upcoming festivities, winter season and mass gatherings, inventive containment strategies need to be implemented by States,” Dr. Bhargava emphasised.
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The country wide prevalence of the virus was similar to that seen in the United States, which was around 9.3%. Brazil and Spain had a prevalence of 2.8% and 4.6%, the ICMR said.
A scientist connected with the survey but who declined to be identified, said the final peer-reviewed version of the study was likely to be available in a month. The lower rate of confirmed cases to true infections was due to an increase in testing as well as the large scale deployment of rapid antigen tests (these are quick tests to determine the presence of virus but also tend to substantially miss infections).
“The opening up of the country has no doubt seen the virus spread and it is quite likely that these numbers will increase in the coming months. There needs to be a consistent decline for several weeks before we can think of having reached a peak,” said the scientist.
The average prevalence in major cities in India ranged from 50% in Mumbai to 29% in New Delhi, 22% in Chennai and 7.8% in Indore. The sero-survey, because it aims to capture national prevalence, samples many more from rural India than cities to reflect the population spread in the country. On May 3, there were 49,720 confirmed cases in India and 3.7 million ones by September 1 — a 74-fold increase. As of Tuesday, India has recorded 6.1 million confirmed infections, of which about 9,50,000 are active cases.
