Survey of India’s 9th-largest city finds COVID antibodies in 52% of the population

More than half the residents in one of India’s largest cities may have developed antibodies for the coronavirus, according to the results of a serological survey released this week.

The survey was carried out in Pune, a city in the western Indian state of Maharashtra with a population of 4 million. Researchers tested 1,664 participants’ blood samples for the presence of coronavirus antibodies, the proteins that the body produces in response to a coronavirus infection. They detected antibodies in 51.5% of the blood samples.

The participants hailed from five areas of Pune; in one of the areas, 65% of participants had coronavirus antibodies.

Subscribe to Eastworld for weekly insight on what’s dominating business in Asia, delivered free to your inbox.

The results indicate that a much higher proportion of people in Pune may have been infected with the coronavirus, and recovered from the disease it causes, COVID-19, than official numbers show.

India’s coronavirus tally as of Friday was nearly 3 million confirmed cases and almost 55,000 deaths, putting it behind the U.S. and Brazil as the third hardest-hit nation in absolute numbers.

If the serological survey’s indications are accurate, the actual number of cases in Pune would be 20 times as high as the official number.

Previous serological studies in other parts of India found comparably high antibody levels among the surveyed population, suggesting that India’s official case number is much lower than the actual number of people infected.

A serological survey conducted in Delhi in the first week of August found that 29.1% of Delhi’s population had coronavirus antibodies—an increase of more than six percentage points since the last serological survey of the city in early July. The earlier survey indicated that Delhi’s case count was 40 times as high as its confirmed case number.

One July survey of people across Mumbai, including residents of the city’s densely packed slums, found that 57% had coronavirus antibodies, prompting one scientist to say that those areas “may have reached herd immunity.”

Reaching herd immunity means enough people in a given area have contracted a disease, recovered from it, and developed immunity to it that the disease stops spreading. It is a controversial approach to the coronavirus pandemic because, in allowing the disease to spread unchecked, it puts more vulnerable groups, like the elderly and immunocompromised, at greater risk of death.

It’s also not yet known exactly how long immunity to the coronavirus lasts. One of the research groups involved in the Pune serological study cautioned that the survey results didn’t account for potential reinfection.

“[The] study does not provide information on immunity from subsequent infection,” the Indian Institute of Science Education and Research (IISER) Pune, one of the groups involved in the research, wrote on Twitter.

Subscribe to Well Adjusted, our newsletter full of simple strategies to work smarter and live better, from the Fortune Well team. Sign up today.