NEW DELHI - A survey by the Indian Council of Medical Research (ICMR) that found that two-thirds of the people in 70 districts had acquired antibodies against Covid-19 has sparked a debate on herd immunity.

The ICMR serological survey, which provides an indication to epidemiologists and the authorities on how widely the virus has spread and is extrapolated to the situation across the country, was conducted in June and July.

Involving 29,000 participants, it found that 67.6 per cent had antibodies, compared with 24.1 per cent when a similar exercise was carried out from last December to January ahead of a deadly second wave of the pandemic that hit the country.

Despite the jump in number, though, epidemiologists and the authorities believe India is no closer to herd immunity as the threshold has been pushed up by the far more transmissible Delta variant of the coronavirus. The Delta variant was responsible for the surge in cases between April and May which, at its peak, saw over 400,000 infections in a day in the country. The number of deaths also soared.

"The concept of herd immunity is not applicable as we are still trying to understand the virus and not when the world is in mid-pandemic. When the pandemic is over and the disease endemic, we will have a far better understanding of herd immunity," said Dr Chandrakant Lahariya, an epidemiologist and co-author of Till We Win: India's Fight Against The Covid-19 Pandemic.

"It also depends on the transmissibility of pathogens. The higher the transmissibility of the variant, the higher the threshold of herd immunity," he added.

Earlier, herd immunity was estimated to have been possible when 60 per cent to 70 per cent of the population were vaccinated or had recovered from Covid-19. But the Delta variant has pushed this threshold to 85 per cent to 90 per cent, according to experts.

Many scientists around the world have also concluded that herd immunity will be tough to achieve for a number of reasons, not least of which is the fact that children, who form a sizeable section of the population in younger demographics like India's, are not being vaccinated. There is also the possibility of far more transmissible variants emerging and immunity levels dropping over time.

"We know antibodies start declining and protection is thought to be linked to that. It will be a rolling journey... That way, we are unlikely to reach that kind of threshold," said Dr Lahariya.

"Clearly, the idea of herd immunity is far away. Covid-19 appropriate behaviour and vaccination are the only ways to fight the pandemic," he said.

Experts are also keeping a close track on what is happening in Israel, where infections are rising in spite of a high vaccination rate.

Israel, which relied on the Pfizer-BioNTech vaccine, has fully vaccinated 58 per cent of the population since January but studies in the country have pointed to a fall in immunity levels. Last week, it was reported that Israel would begin offering a third dose of the vaccine to people aged over 60, a world first in efforts to slow the spread of the Delta variant.

The drop in immunity levels would be worrisome for a populous country like India, which thus far has vaccinated just over 9.8 per cent of the population. Unlike Israel, India also may have to contend with regional variations, according to the ICMR survey, making it tougher to build immunity evenly across the country.

The survey found 70 per cent of the people in the states of Bihar and Odisha with antibodies, but the figure was 44.4 per cent in Kerala.

Some experts, such as leading virologist Dr Shahid Jameel, remain sceptical about the survey results.

Dr Shahid is not convinced that they can be representative of India, which has a massive population of 1.35 billion people.

"The 29,000 people sampled in 70 districts may not be truly representative of a country of India's size and complexity. Only 10 per cent of districts were sampled - India has 742," he said.

He also noted that the survey indicated that 400 million people were not exposed to the virus - enough to fuel fresh outbreaks and therefore there was a need to protect them.

There appears to be general consensus, though, among the experts that India will not be hit by a devastating third wave of Covid-19.

"The third wave (if there is ever a full-fledged wave) would be milder than the second, at least in terms of severe disease, hospitalisation and mortality," said Dr Shahid.

Officials, in the meantime, have continued to warn people against laxity as the economy opens up, saying the rising number of cases around the world has shown that the pandemic is far from over and that there are worrying trends in India too.

"There is a consistent decline seen in cases, but in the past few weeks the decline has slowed and that remains a concern," Mr Lav Agarwal, a top Health Ministry official, told a briefing on Friday (July 30).