COVID-19 antibodies found in 57% of Mumbai slum residents in limited serosurvey by NITI Aayog, BMC, TIFR

In a serological survey of roughly 7,000 people in Mumbai, researchers in their first round of surveys found COVID-19 antibodies present in 57 percent of participants from slums, and 16 percent of participants from non-slum areas. The survey is joint effort by NITI-Aayog, the Municipal Corporation of Greater Mumbai (MCGM) and the Tata Institute of Fundamental Research (TIFR), Mumbai.

In the first round of antibody surveys, 6,936 volunteers were chosen from slums and non-slum areas, and their blood tested for the presence of SARS-CoV-2 antibodies. Researchers intend for the study to be representative of the larger population in the city, and reflect how common antibodies against the SARS-CoV-2 virus are in the general population.

The survey used a random sampling method to pick out volunteers from three municipal wards in Mumbai: R-North (Dahisar), M-West (GM Link Road to Suman Nagar Junction; Mahul to Somayya Nallah) and F-North (Matunga, parts of Thane Creek and Mankikar Marg).

Volunteers chosen were a random selection of people of different ages & genders, and a mix of people from the general population as well as health care workers. Samples were collected from each volunteer at two different time points, to understand the pattern of spread in a large sample size over a period of time. The results from the first round of antibody surveys were made public in a widely-shared report from the Health Ministry on 27 July.

Researchers found that antibodies against the COVID-causing SARS-CoV-2 virus were in 57 percent of samples from slums, and 16 percent in those from non-slum areas, on average. More women in the sample population had the antibodies that men. The age data from various wards didn't suggest a higher prevalence of antibodies in any particular age category.

Researchers have inferred from their findings that a high proportion of all infections are likely to be asymptomatic. The higher prevalence of antibodies in slums could be because of the higher population density and common facilities shared between many people, including toilets and water pumps.

While the COVID-19 case fatality rate (CFR) is roughly 5-6 percent, researchers think the infection fatality rate (IFR) is probably very low (under 0.1 percent), based on the antibody serosurveillance data and records from BMC on reported deaths. This could also be a reflection of effective efforts to contain active cases and isolate symptomatic ones by the municipality, they said in the first report.

Researchers suggested that better social distancing and access to better hygiene as well as interventions by the local municipality to freeze the spread of infections were likely explanations for the lower antibody prevalence in non-slum areas surveyed.

"These results will be valuable to learn more about herd immunity," the report reads. "Although it is still unclear what level of prevalence leads to herd immunity, findings indicate that at least in slums, this could be attained sooner than later, if the immunity exists and persists in a significant proportion of the population."

The data also suggested that social distancing and precautions such as wearing masks and maintaining hand hygiene are effective methods to slow the infection spread. The study recommend the continued use of these measures as a new normal in all sections of the society, regardless of COVID-19 prevalence.

The study is ongoing, and researchers have planned further analysis on how prevalent neutralizing antibodies are in the population, and the risk factors for a SARS-CoV2 infection. Repeat surveys have been planned to make more inferences about herd immunity in the population of Mumbai.

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