Mumbai is home to around 1.2 crore people, of which some 65 per cent live in slums.
A medical survey of nearly 7,000 people in Mumbai has found that one in six or about 16 per cent of residents in the city had contracted the coronavirus. In slum areas, where lakhs of people live in cramped spaces usually sharing toilets, the number was a whopping 57 per cent.
The serological surveillance study used random sampling in the first two weeks of this month in the city's Dahisar, Chembur and Matunga areas. In sero studies, doctors test the blood of a section of the general population to check the presence of anti-bodies to any disease.
Anti-bodies develop only when the person had contracted the disease in the past. It not only indicates how far among the general population the disease has spread, but whether the people are moving towards herd immunity.
India's financial and film capital, Mumbai has now recorded over 1 lakh cases - over 7 per cent of the country's tally - with more than 6,000 deaths attributed to the coronavirus.
The city is home to around 1.2 crore people, of which some 65 per cent live in slums. A further 60 lakh people are estimated to live in peripheral districts.
The study was conducted jointly by the NITI Aayog, the Municipal Corporation of Greater Mumbai and the Tata Institute of Fundamental Research. Samples were collected from volunteers in three municipal wards. The survey found the prevalence of coronavirus anti-bodies marginally higher in women.
Authorities say the results hint towards early signs of herd immunity - when the spread of a disease is stopped naturally when enough of the population becomes resistant to the virus and not enough people are able to transmit it.
"Heading towards herd immunity. We will conduct a second round of survey in August and results to be out in September," Suresh Kakani, additional municipal commissioner, said.
Medical experts, however, caution against letting the guard down. "Anti-bodies stay for 3-6 months in your body. Then they weaken. This will not create herd immunity. In fact, unlocking is happening now and if local trains start, then a second wave of coronavirus can come," said Dr Avinash Bhondve, President of the Maharashtra Medical Association.
The study also found that a large number of infected people were asymptomatic and points to a fatality rate as low as 0.05 to 0.10 per cent.
Mumbai recorded 717 new cases of coronavirus on Tuesday, its lowest one-day rise in more than two months, taking the tally to 1,10,846, while 55 more patients died taking the total to 6,184, authorities said.
Last week, a sero-prevalence study in Delhi found that 23.48 per cent of the people were affected by COVID-19 in the city. The survey also indicated that a large number of infected persons remained asymptomatic, increasing the chances of the spread.