Mumbai: Sero-survey finds antibodies in 57 % of those tested in slums

Mumbai’s first sero-survey has revealed presence of antibodies in 57 per cent of those surveyed in slum pockets and 16 per cent of those tested in non slum middle-class and upper middle-class neighbourhoods.

The BMC health workers, in conjunction with Niti Aayog and Tata Institute of Fundamental Research tested 8,870 people across three wards – M (west) covering Chembur and Mahul; F-north covering Matunga, Sion, Wadala, Dadar and parts of Prabhadevi; and R north covering Dahisar.

The presence of antibodies indicates that the person contracted the infection, fought it off and in the process developed reasonable resistance to the virus. The larger the number of people in a community with antibodies the better because it leaves virus with a narrower scope to spread.

At the same time, a lower incidence antibodies – as in slightly better off neighbourhoods in this survey – is also not a bad thing as it indicates fewer people contracted the infection. This means that precautions like social distancing, use of masks, and maintaining hygiene worked in these areas.

Sero-surveys not only help communities understand how effective their fight against an infection has been, they also help in formulating future action plans, interventions and resource distribution.

Since this sero-survey used random samples, a lot of those tested and found to carry anti-bodies may not even know that they were infected. This is because many infected with Covid-19 remain asymptomatic.

Additional Municipal Commissioner Suresh Kakani described the results of the sero-survey as encouraging and said the report indicates that Mumbai is on the right track. “Timely transfer of symptomatic patients from community to isolation centres has worked,” he said.

Asked if the presence of antibodies in 57 per cent of those surveyed in slum pockets indicated that slums in Mumbai may have developed “herd immunity”, Kakani said that the researchers were reluctant to draw such a conclusion before the report is shared with national and international agencies.

Herd immunity refers to a large part of a community developing resistance to an infection, either by developing anti-bodies naturally or through an immunisation drive, leaving the virus with fewer bodies to infect and eventually petering out.

Kakani said that the survey revealed that infection fatality rate in slums is now down 0.05 to 0.10 per cent. “That is bare minimum and a very encouraging indicator. We felt deaths in slums would be more. But it wasn’t so,” he said.

Another finding of the sero-survey was the marginally higher prevalence of anti-bodies among women. The survey also indicated a lower rate of infection among women at 45 per cent against men’s 55 per cent. “Women – and it has been proven beyond doubt in national and international studies – have greater immunity against Covid-19. This is because of oestrogen hormone. Women also developed antibodies faster than men,” Kakani said.

Antibody testing involves drawing blood to test serum for antibodies against Covid-19. If a test is positive, it indicates that the person has been exposed to the infection around 15-21 days prior to the test. The test looks for IgG antibodies. These antibodies are produced in the body at least 14 days after the infection.

Kakani said the low prevalence of infection among middle class neighbourhoods could be attributed to social distancing, use of masks and access to better hygiene. “The higher sero prevalence in slums could possibly be due to population density and shared common facilities like toilets,” he said.

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