Two-thirds of India's more than 1.3-billion-strong population have antibodies against the coronavirus, according to data released recently from a serological survey conducted by the country's top medical body, the Indian Council of Medical Research (ICMR).  

The survey was conducted nationwide in June and July and involved 29,000 participants. It also for the first time included 8,691 children aged six to 17 years. Half of them were seropositive, meaning that they had been exposed to the virus.

Among the adults participating in the study, 67.6% were seropositive, while more than 62% of adults were unvaccinated. As of the end of July, nearly 7% of eligible adult Indians had received two vaccine doses.

The study also surveyed 7,252 healthcare workers and found 85% had antibodies, with one in 10 not vaccinated.

Nevertheless, the survey showed, about 400 million of India's 1.4 billion people were lacking antibodies.

Serological surveys provide data on the proportion of population exposed to the coronavirus, including asymptomatic individuals, through the presence of antibodies that typically start appearing about two weeks after of the onset of infection.

While RT-PCR and rapid antigen tests look for the presence of the actual virus, the antibody test checks for antibodies in the blood. So-called seroprevalence indicates frequency of the presence of such antibodies among individuals.

"From the first sero-survey in May and June 2020, when one in 140 adults had antibodies for the virus, two in three Indians now have antibodies either because of prior infection or vaccination,” renowned virologist Jacob John told DW.

Limited reliability

However, epidemiologists and scientists caution that such serological studies are not always reliable and credible.

"The ICMR sero survey examines only 70 out of the 700-odd districts in India, across 21 states. Because less than 10% of India's districts are sampled, it is possible that we may miss large variations between districts," Gautam Menon, professor of physics and biology at Ashoka University, told DW.

"It is here that we need more information from well-planned sero surveys across different regions of the country, so that we have a much more granular view of how the pandemic is proceeding," he added.

The uneven spread of the COVID-19 infection complicates the task of sampling a population to determine how many have been infected. Experts say the timing, the choice of the antibody test and the sampling methodology can have huge effects on the results.

Vineeta Bal, a scientist at the National Institute of Immunology, is among experts who believe that while surveys like this can be useful, they have their limitations.

"It is difficult to say how representative the survey is of various parts of the country because the infection and incidence rates vary across states," Bal told DW.

She said that for example, it was unclear if seropositivity was higher in the states that had higher infection rates.

Scientists also argue that there is a need for more data from the district level as well as information about reinfections and the problem of viral immune evasion, in order to put place strategies to manage the impact of COVID-19 in the future.

Still vulnerable to COVID-19

Moreover, experts say that antibody prevalence does not necessarily mean that the population is less susceptible to new infections. 

Recent studies have shown that despite the high seroprevalence in countries like the United Kingdom and Israel, those countries have seen a recent rise in infections.

"Look what happened in Manaus, a Brazilian city of more than 2 million. It developed supposed 76% herd immunity by October last year, but the city was besieged by a second wave subsequently," Brahmar Mukherjee, a professor of global health at the University of Michigan, told DW. "This was also the case in pockets of Delhi and the Mumbai slum areas," she added.

Mukherjee pointed out that serological studies should not lead to complacency also because the virus is changing and there are cases of reinfections, especially with the delta variant.

"We can only be secure with vaccinations, and that is the way forward, along with COVID-appropriate protocols," she said.

Seroprevalence data from around the country has deepened the understanding of the COVID crisis in India — but given that a huge swathe of the population is not vaccinated, this could still create conditions for new variants to emerge.

India's total cases currently stand at 31.4 million, according to Health Ministry data, making it the second-most affected country after the United States. The South Asian nation has recorded more than 420,000 COVID-related deaths so far.