Residents queueing to be tested for Covid-19 at the Dharavi slum in Mumbai. Photo: AFP/Indranil Mukherjee

Mumbai’s slums could be close to attaining herd immunity to Covid-19 if current trends of infections are sustained, a survey measuring the prevalence of the disease has shown.

Similar studies on identifying antibodies among the population could help predict and contain the virus that has spread across India to infect at least 1.64 million people. That’s the third-highest Covid-19 tally in the world, trailing only the US and Brazil.

Mumbai municipality conducted a survey in the first half of July on 6,936 people across three of the city’s 24 wards. It found 40.5% of those tested had been exposed to Covid-19 and had developed antibodies against it.

As many as 4,234 of those tested resided in slums and 2,702 lived in non-slum areas. The virus level in the slum population was 57% compared with 16% in non-slums where issues such as social distancing, mask-wearing and cough and sneeze hygiene was less of a challenge.

It is generally believed herd immunity can be achieved from 60% of a population or more developing antibodies. Herd immunity happens when a large part of the population becomes immune to an infectious disease and indirectly protects the others.

A survey in New Delhi conducted between June 27 and July 10 and involving 21,387 people showed that 23.48% had developed antibodies.

Both surveys revealed a high number of asymptomatic infections as a proportion of all cases.

Of the 1.64 million cases in India, 1.06 million people have recovered. The number of those in critical condition has remained below 9,000. India’s fatality rate is 2.21%, lower than most other countries.

But even as infections rise and spread across more states, the government is under immense pressure to relax social distancing and reopen the economy. This week, authorities gradually relaxed lockdown restrictions on more activities, allowing for the re-opening of gymnasiums and yoga centers and lifting of night curfews.

More controversially, they have also eased restrictions on the movement of people across states. They have retained curbs on the opening of metro trains, schools and colleges, cinemas, theaters and all large gatherings. Maharashtra has allowed the opening of shopping malls and other activities to encourage a pickup in economic activity and employment.

At the same time, the virus is spreading into new areas including the southern states of Andhra Pradesh and Karnataka. Cases have surged from 15,000-odd in the beginning of July to more than 120,000 in Andhra Pradesh. Officials attribute the rise to higher rates of testing. Karnataka and Andhra Pradesh are second and third in terms of active cases after Maharashtra.

On a national level, India has been ramping up its testing facilities and numbers. India now tests half a million a day and aims to double that number before the end of September.

Yet it’s been a see-saw battle for most Indian states, including Karnataka and Kerala in southern India, which managed to keep a lid on the virus during the initial phases of India’s outbreak.  

Karnataka has more than 110,000 cases, including a rapid increase in infections in the state capital of Bangalore, and Kerala more than 21,000 from barely a few thousand previously.

Still, a health ministry official said India is not treating herd immunity as a strategic choice or an option because of its large population size of 1.38 billion. It would mean being prepared for a large number of fatalities.