A survey of 6,936 people living in three Mumbai districts found that 57% of the people tested positive for coronavirus antibodies, the proteins produced by the body’s immune system to fight off disease and an indicator that a person has been infected by and recovered from COVID-19, the disease caused by the coronavirus.

The surveyed areas included some of India’s biggest slums, including one slum, Dharavi, where one million people reside and the population density is more than 10 times that of Manhattan.

According to the survey results, the death rate in the surveyed areas, where the population skews young, is between 0.05% and 0.1%. Mumbai’s overall death rate is currently around 5.6%, with over 110,000 confirmed cases and 6,180 recorded deaths.

“Mumbai’s slums may have reached herd immunity,” said Jayaprakash Muliyil, chairman of the Scientific Advisory Committee of India’s National Institute of Epidemiology.

Herd immunity is an approach to the coronavirus pandemic where, instead of instituting lockdowns and other restrictions to slow infections, authorities allow daily life to go on as normal, letting the disease spread. In theory, enough people will become infected, recover, and gain immunity that the spread will slow on its own and people who are not immune will be protected by the immunity of those who are.

University of Chicago researchers estimated in a paper published in May that achieving herd immunity from COVID-19 would require 67% of people to be immune to the disease. Mayo Clinic estimates 70% of the U.S. population will need to be immune for the U.S. to achieve herd immunity, which can also be achieved by vaccinating that proportion of a population.

The high level of immunity in the surveyed areas in Mumbai was not by design. People in Dharavi, for example, live in extremely close quarters where social distancing is often functionally impossible—families of eight live together in 100-square-foot rooms and up to 80 people share use of a single toilet.

Deliberate herd immunity is a controversial approach because it puts people most vulnerable to COVID-19—the elderly and immunocompromised—at a greater risk of death. The U.K. government considered the strategy in March, but changed tack when the country’s death toll started climbing, drawing public criticism.

Sweden chose to pursue the strategy, and it’s recorded a higher death rate than neighboring countries that imposed lockdowns. Around 14% of people in Sweden’s capital, Stockholm, had coronavirus antibodies in May, much lower than the share recorded by the Mumbai survey, which was conducted by municipal authorities and the Tata Institute of Fundamental Research, a public research university in Mumbai.

Coronavirus cases in India are currently growing at a faster rate than anywhere else in the world, but the slums where the survey was conducted have recorded steep drops in infection numbers in the last few weeks. Separate surveys of people in New Delhi and Mumbai recently have estimated that around a quarter of the total population of each city may have coronavirus antibodies.

Some parts of New York City may have reached levels of immunity similar to those recorded in the Mumbai survey. Over 68% of people at a clinic in one Queens neighborhood had coronavirus antibodies. It’s still unknown how those antibody levels will affect infection rates in the event of a second wave in New York.