New Delhi: More than half of Mumbai’s slum population has already been exposed to Covid-19 virus and has developed antibodies against it, a sero-surveillance survey conducted by the Brihanmumbai Municipal Corporation (BMC) has found.
The survey, conducted in the first week of July by the BMC, NITI Aayog and the Tata Institute of Fundamental Research (TIFR) found 57 per cent of the samples from slums and 16 per cent from non-slums had Covid-19 antibodies.
A serosurveillance survey is held to know the presence of antibodies against the virus in a population, revealing the exposure of the infection in the community.
More than 6,936 samples were collected from the slum and non-slum populations of the three wards of Dahisar (R-North), Chembur (M-West) and Matunga (F-North), representing the city and its eastern and western suburbs that together have an almost 18 lakh population.
Since 42 per cent of the Mumbai residents live in slums, the results mean a large number of the people have already been infected by the virus and may have developed immunity against it.
“This is probably the first serosurveillance survey that found such a high prevalence of Covid-19 antibodies in the world,” said Dr. Ullas Kolthur-Seetharam, Professor, Department of Biological Sciences, TIFR. “The results of this study will be useful to understand herd immunity and disease transmission.”
BMC authorities are seeing the results as a vindication of their strategies.
“This shows that our containment strategies worked and that 57 per cent of the slum population have already developed antibodies that will protect them against the virus at least for 2-3 months,” said Suresh Kakani, Additional Municipal Commissioner, BMC.
Death rate lower than expected
The results suggest that there is a higher proportion of asymptomatic than symptomatic infections, said the press release by BMC.
“Taking together the current prevalence (estimated here) and records from BMC on reported deaths, the infection fatality rate (IFR) is likely to be very low (0.05-0.1%),” it further said.
“Among others, this could be attributed to effective containment efforts and active measures to isolate symptomatic cases by MCGM (Municipal Corporation of Greater Mumbai),” the release read.
Higher prevalence of antibodies in the slums is likely because of population density and shared common facilities like toilets and water points, said the release.
Not sure about herd immunity yet
Would the large number of the population exposed mean they are at lower risk? The researchers did not say.
“Although it is still unclear what level of prevalence leads to herd immunity, our findings indicated that at least in slums it could be attained sooner than later if the immunity exists and persists,” the press release said.
It is too early to say whether Mumbai has developed herd immunity and what numbers are required to reach this level, said Kolthur-Seetharam. “We will be conducting neutralising assays on the samples to know if the antibodies developed by people are able to fight the virus,” he added.
The study shows that the virus has spread widely in the slum population with only 44 per cent unexposed as compared to 84 per cent in non-slum areas.
The difference between the two groups show the important role population density, differences in socio-economic factors and social distancing play in controlling the spread, said Kolthur-Seetharam.
Large number of asymptomatic patients spread the virus
While both the Indian Council of Medical Research and the BMC focussed on testing symptomatic patients and isolating them, the survey shows the infection of the virus has been far wider than previously thought of.
Kolthur-Seetharam said while they are not sure whether asymptomatic people have a lesser chance of spreading the virus, the strategy to test the symptomatics was effective. “Since its not possible to test the entire population, the focus was to reduce fatalities by isolating the ones that need medical attention, which is a good one,” he said.
While in the early stage of the pandemic, 70 per cent of the cases came from the slums, and 30 per cent from the non-slums, this has been reversed now. “We have now changed our strategy to focus more on buildings than slums,” said Kakani.
The low prevalence in non-slum areas mean social distancing, wearing face covers and hygienic measures like hand washing help, said Kolthur-Seetharam.
This was just the first phase of the survey, which will include a follow up of the same population after a month in the second phase to understand trends in disease transmission, he said.