NEW DELHI - A recent survey in Mumbai's slums finds that more than half of the residents have developed antibodies for COVID-19, indicating that some of India's most densely packed settlements, which pose the greatest challenge in containing the spread of the infection, could be heading toward "herd immunity."
While urban slums may be inadvertently developing population immunity, though, officials are ruling it out as an option in tackling the rampaging infection, saying it could never be a choice but only an outcome.
"Herd immunity" is achieved when a large part of the community develops antibodies against the virus, acting as a wall against its further spread.
India is a global hotspot for COVID-19, with infections topping 1.6 million.
The study in Mumbai found that compared to 16 percent of people living in more affluent parts of the city, 57 percent in three slum clusters had been exposed to the infection and tested positive for antibodies.
It means the virus has raced more swiftly through cramped slums where as many as eight to 10 people pack into tiny rooms, making physical distancing impossible.
But on the plus side, most of these people were either asymptomatic or had shown very mild symptoms – thus they were never tested earlier and not reflected in official numbers.
"It is safe to assume that slums could reach herd immunity sooner than later, but the big question is how long does the immunity persist," says Ullas Kolthur, a professor at the Tata Institute of Fundamental Research, one of the scientists who carried out the study among nearly 7,000 people.
Although Mumbai is one of India's worst-hit cities, COVID-19 numbers have begun to diminish.
Even more encouragingly, infections in the city's slums that pack in 5 million people have shown a marked decline in recent weeks, giving a breather to authorities.
The serological study that uses antibody tests to look for an immune response was carried out by Mumbai's civic authorities, a government think tank, Niti Aayog, and the Tata Institute of Fundamental Research.
"It indicates that population density does affect the spread of COVID-19," says Kolthur. "It could be due to shared toilets or shared facilities, as long as there is more intermingling of the population, the spread is high."
In Mumbai officials said they were surprised by the apparently high numbers of asymptomatic people.
Researchers also point to the infection mortality rate of less than 0.05% based on the official number of deaths in the slums.
"The slum people seem to be having a much lower fatality rate compared to non slums. That is also very surprising, one would have expected it to be the other way around," points out Kolthur. "It could be due to a slightly younger population that probably has more resilience. It needs to be further investigated."
A government survey carried out earlier in July in parts of the capital New Delhi found that one in four residents had developed antibodies.
As the two surveys turn the spotlight on possible "herd immunity," the health ministry said that in a country like India with a huge population, this can only be an "outcome" and that, too, at a very high cost as it means lots of people would have to be infected.
Rajesh Bhushan, officer on special duty in the health ministry underlined that the country must follow "COVID-10-appropriate behavior"—like wearing masks, avoiding gatherings and following hand hygiene util a vaccine is developed.
After being ravaged by the coronavirus, both Delhi and Mumbai are showing a downward trend.
Nonetheless, Mumbai, like several other parts of the country, continues to impose a stringent lockdown. Some commentators said the results of the serological studies test the logic of the shutdown and should encourage authorities to restart economic activity.
Pointing to the recent surveys in the two cities, The Times of India newspaper said in an editorial that the "high prevalence of antibodies with asymptomatic spread makes a strong case for faster unlocking with safeguards."