A sero-survey conducted in Mumbai showed that more than half the people, or 57 per cent, tested in slums had been exposed to and developed antibodies against SARS-CoV-2 virus as compared to only 16 per cent of those tested in residential societies. The survey, conducted by the Brihanmumbai Municipal Corporation in three wards of Mumbai, was done on 6,936 people – 61 per cent of them slum-dwellers and the remaining 39 per cent in residential societies.
The survey also found that more women than men had developed antibodies. Among people in slums, 59.3 per cent of the2,297 women tested, and 53.2 per cent of the 1,937 men tested had antibodies against Covid. In non-slum areas, 16.8 per cent of women tested and 14.9 per cent of men tested had developed antibodies.
However, until now, RT-PCR tests in Mumbai had showed more men (55 per cent) getting infected than women (45 per cent).
Overall, the survey found that 40.5 per cent had been exposed to coronavirus in Mumbai’s Matunga, Chembur and Dahisar areas, where the sero survey was conducted in the first fortnight of July. At 57.8 per cent, Matunga, which is close to Dharavi, had the highest exposure rate.
Officials who conducted the survey said shared facilities such as toilets, high population density and lack of physical distancing may be among reasons why those living in slums were 3.5 times more exposed to the virus than people in housing societies.
The high exposure in slums indicates that fatality rate in Mumbai may be “as low as 0.05-0.10 per cent”, instead of existing 5.5 per cent, the survey observes.
Despite the high exposure in slums, the survey found that fatalities had not been high. “This could be due to multiple factors… this could also be due to a possibly higher percentage of younger population in slums,” said Dr Ullas Kolthur Seetharam from the Tata Institute of Fundamental Research (TIFR), which conducted the survey along with BMC and NITI Aayog.
None of the nearly 7,000 people tested had taken an RT-PCR test for Covid-19, which either means they were asymptomatically infected, or they may not have had serious enough symptoms to suspect an infection.
“This is a surprise, more people remain asymptomatic than we expected,” said Suresh Kakani, Additional Municipal Commissioner.
On whether the high presence of antibodies is indicative of herd immunity, Maharashtra state epidemiologist Dr Pradeep Awate, said, “Development of only IgG antibodies is not immunity, a body needs to develop cellular immunity. But yes, slums will attain herd immunity faster than the general population.”
Awate said if this data is extrapolated to the larger population, 50-70 lakh may have already been exposed. “But we should get appropriate geographical representation of all 24 wards of Mumbai to reconfirm.”
On women developing more antibodies than men, clinical scientist Dr Gagandeep Kang said, “We need to repeat the survey over time and in other areas to see if the trend continues… While infection occurs similarly in men and women, oestrogen in women protects them from disease.”
TIFR and BMC officials claimed the sample for the sero survey – which included slum-dwellers and others, men and women, young and old – was broadly representative of the population, unlike antibody tests done by private labs, covering who could afford to get tested.
BMC officials said that a second round of sero survey will be conducted a month later.