The findings of a serological survey by the Indian Council of Medical Research (ICMR) from 69 districts across 21 States indicate that an estimated 7,00,000 people could have been infected by the SARS-CoV-2 virus in these districts even in early May.
The number indicated by the survey is twenty times higher than the 35,000 confirmed cases of COVID-19 reported as of early May in the entire country, suggesting that the actual COVID-19 count overall could be underestimated by a factor of at least 20.
Confirmed infections have since swelled to more than 3,08,000 as of Friday (June 12), with more than 8,900 people having died of the disease.
Tested for antibodies
According to a research paper published in May in the ICMR’s in-house Indian Journal of Medical Research describing the methodology of the survey, blood samples of 24,000 adults were examined for antibodies produced specifically for SARS-CoV-2 via an ELISA test .
On Thursday, Director General of ICMR Balram Bhargava at a press conference said 0.73% of the “population in the districts surveyed had evidence of past exposure to the virus.” The actual number of those infected was not disclosed. In other words, the survey revealed that 0.73% of those sampled showed evidence of IgG antibodies.
The survey began around May 12 and given that it takes an average of two weeks for IgG antibodies to be detected, April 30 was considered to be the date on which the individuals were likely infected by the virus.
The adult population of these districts according to the 2011 census was 60% of the overall population (160 million) and this works out to around 96 million persons.
If the results of the survey are applied to the adult population of these districts, the total number of people likely to have infected by the virus adds up to 7,00,000; the numbers could be even higher if the growth of the population in the last nine years is factored in. The Hindu corroborated these findings with a scientist, who didn’t want to be identified, but was part of the survey design and analysis.
Antibodies produced in response to being infected by the virus confer immunity but are also evidence of being exposed to the infection. Because they rely on blood samples and are only an indirect evidence of the presence of the virus, they aren’t as accurate as PCR tests. Two values called sensitivity and specificity — that are a measure of the proportion of cases mis-identified or outright missed — determine the accuracy of the test.
Academic literature has suggested that there is a higher chance of “false positives” being reflected in serological surveys if there is a low prevalence of the disease. The survey measured prevalence in regions that had highly variable incidence of the disease and 7,00,000 infections is an average of a wide range — the details of which, again, aren’t yet available — of infection values.
The scientist that The Hindu spoke to said while the prevalence percentage accounted for the limitations of this test, 7,00,000 was an “accurate estimate” of the level of infection in these districts. The test has been developed and validated by the ICMR-National Institute of Virology and is said to be the most reliable so far.
The ICMR is expected to detail, in the coming days, more fully the results of its sero-survey.
Levels in hotspots
A separate aspect of the survey was to gauge levels of infections in hotspots for which another set of districts and cities were evaluated..
“The test has limitations in that its accuracy is dependent on background levels of the disease in a region. This test is good, it could have been better but works well for the purpose of detecting infection in the community,” said Dr Jayaprakash Muliyil who was one of the epidemiologists involved in the design of the survey and is a Member of the National Task Force-constituted research committee on (COVID-19) Epidemiology and Surveillance.
“Community transmission has been evident for long. I haven’t seen the analysis but it is not surprising that the actual infections are much larger. It’s unlikely India would have a different trajectory from any other place,” he added.
Another expert familiar with serology surveys in India seconded The Hindu’s calculations. “Twenty cases for every single confirmed one is reasonable for a country India’s size and does show a fair degree of containment. However, these true infection numbers ought to have been disclosed by the ICMR,” the person added.
The Hindu reached out to Dr. Bhargava for his comments, but couldn’t get a response.