While the number of new COVID-19 cases in the city’s urban slums is decreasing, a survey suggests that this is due to an efficient early spread of the virus

  • By Ari Altstedter and Dhwani Pandya / Bloomberg

About six in 10 people living in some of India’s biggest slums have antibodies for COVID-19, indicating that they have recovered from infection, in what could be one of the highest population immunity levels known worldwide.

The findings, from a serological survey that took samples from 6,936 people across three suburbs in India’s financial center of Mumbai last month, might explain why a steep drop in infections is being seen among the closely-packed population, despite new cases accelerating overall in the hard-hit country.

“Mumbai’s slums might have reached herd immunity. If people in Mumbai want a safe place to avoid infection, they should probably go there,” said Jayaprakash Muliyil, chairman of the Scientific Advisory Committee of India’s National Institute of Epidemiology, and the retired head of one of India’s premier medical colleges.

The findings of the study, which was conducted by municipal authorities and the Tata Institute of Fundamental Research, suggest that despite efforts to contain the spread of the disease, Mumbai’s poorest neighborhoods might have unwittingly pursued the controversial strategy of attaining herd immunity. This describes an approach in which infection is allowed to run through a population to faster neutralize the pathogen’s threat.

The idea is controversial not only because of the risks to those who get infected, but because there is still debate over how much immunity an infection leaves and how long it lasts. Some studies have shown COVID-19 antibody counts dropping after only a few months, creating the possibility of re-infection, though there has been little evidence so far that re-infections with the virus have been occurring. Some experts have raised the possibility that other immune cells continue to provide immunity even after antibodies fade.

About 57 percent of the surveyed people in the slums of Dahisar, Chembur and Matunga had antibodies in their blood, compared with 21.2 percent found in an April study in New York City, and 14 percent reported in Stockholm in May.

A “hands-off” approach has been criticized in places like Sweden, where it appears to have resulted in more deaths than in neighboring countries that implemented lockdowns. The findings from Mumbai’s slums, where the population is young and less predisposed to severe cases of COVID-19, might support public health strategies more focused on protecting the vulnerable without trying to suppress the virus completely. The study’s authors assume that asymptomatic cases constitute a large part of all infections in the surveyed areas.

With social distancing more or less impossible, Mumbai’s slums are singularly well-suited for the spread of COVID-19. Dharavi, the city’s largest slum, packs a population as big as San Francisco’s into an area the size of New York’s Central Park, with as many as 80 people often sharing a public toilet, and families of eight regularly sharing a small room.

Yet the slums have seen steep drops in infections in the past few weeks after cases first erupted in April, even as India’s overall cases grow at the fastest pace globally. Credit has largely been given to the intensive containment measures officials implemented in the slums, like door-to-door health screenings and rapidly erected quarantine facilities.

The serological findings suggest another possibility: The crisis might be largely over because the virus has spread efficiently, not because it was stopped.

“One explanation is they did an excellent job containing it, the other is that herd immunity has been reached. The virus does its work. The virus doesn’t worry about your quarantine, and it is much more efficient than your efforts to contain it,” Muliyil said.

To be sure, the findings should not be taken as reason to relax precautions like mask-wearing and maintaining as much physical distancing as possible, Public Health Foundation of India president K. Srinath Reddy said, adding that tests like the ones Mumbai used to conduct its survey have a tendency to throw out false positives, so the actual rate of exposure might be lower.

Even if the numbers are taken at face value, most of Mumbai’s slum dwellers venture regularly into the rest of the city for work or other reasons, putting them at risk of transmission to or from people elsewhere.

“It’s not really herd immunity, it’s herd protection. If you wander out to another herd which has had a lower degree of exposure, there’s no such protection,” Reddy said.

For Muliyil, the government’s containment measures in the slums helped keep mortality rates low, because the proactive surveillance ensured that cases were caught early and given high quality care. Among a population as big as 1 million people, Dharavi has recorded 253 deaths.

Growing immunity might also be behind the dip in cases in New Delhi, said Muliyil, where a study in early July found that a quarter of the population had been exposed.

Epidemiologists generally believe that infection levels must reach 60 percent to create herd immunity. However, if exposure is concentrated in the populations least able to socially distance, like in slums, it could still slow the overall spread of infection.

In Mumbai, new cases dropped to the lowest tally in almost three months this week, although the July antibody survey found that only 16 percent of the population in wealthier areas of the city had been exposed.

Hospitalizations seem to be going down in both of India’s hardest hit major cities, another indication their outbreaks might be easing.

Still, there have been persistent reports of cases and deaths being under-counted around India, raising skepticism over whether infections are truly dropping in both cities. The Delhi government was on Monday censured by a court for basing its numbers too heavily on antigen tests with high rates of false negatives.

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