Last month, researchers in one of India’s largest cities made a surprising discovery. Of the nearly 7,000 blood samples taken from people in Mumbai’s slums, 57% tested positive for coronavirus antibodies.

While some were alarmed by the results of the study conducted by the Mumbai authorities and the Tata Institute of Fundamental Research, others were optimistic.

Mumbai’s slums, where social distancing is nearly impossible, might now have some of the highest levels of immunity in the world — only 23.5% of samples taken by India’s National Center for Disease Control tested positive for antibodies in Delhi and 14% tested positive in New York, in a study sponsored by the New York State Department of Health.

Scientists believe it’s likely that recovering from coronavirus leaves a person with some immunity, but it’s not clear how strong it is or how long it lasts. Herd immunity is the idea that a disease will stop spreading once enough of a population becomes immune — and is appealing because, in theory, it might provide some protection for those who haven’t been ill.

If more than half of people in Mumbai’s slums had contracted coronavirus, could they be approaching herd immunity — without a vaccine? 

One expert thought so. 

“Mumbai’s slums may have reached herd immunity,” Jayaprakash Muliyil, chairman of the Scientific Advisory Committee of India’s National Institute of Epidemiology, said, according to a Bloomberg report. “If people in Mumbai want a safe place to avoid infection, they should probably go there.”

But others have been more cautious. David Dowdy, an associate professor in epidemiology at Johns Hopkins Bloomberg School of Public Health, said it was possible that the researchers had used a test that created false positives. 

And Om Shrivastav, an infectious diseases expert in Mumbai, cautioned that, less than eight months into the virus’ existence in society, it was too early to make any “decisive, conclusive statements.” 

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