People in the 41-60 age group were most exposed to the coronavirus in both slums and non-slums, according to the findings of the second sero-survey in Mumbai. The overall Covid-19 sero-prevalence found in the second survey conducted jointly by the BMC, Tata Institute of Fundamental Research and Niti Aayog is 33.4 per cent, a fall from 40 per cent in the first survey.
Detailed findings published by TIFR show that of 1,060 people aged 41-60 years surveyed in slums, 50.3 per cent tested positive for antibodies against Covid-19, and of 964 people in same age bracket surveyed in non-slums, 18.6 per cent tested positive. This age group also steps out more for work putting them at higher risk of exposure to coronavirus.
After the 41-60 age group, people in the 12-24 age group living in gated societies and high-rises had the most exposure rates. At least 18.5 per cent tested positive for Covid antibodies, followed by those aged 25-40 (16.6 per cent exposure). Those aged above 60 years in non-slums remain most protected and least exposed. Of those aged above 60 surveyed in non-slums, 13.2 per cent had antibodies.
In contrast, in slum areas, exposure and development of antibodies was high in those aged above 60 years. At least 48.2 per cent of those aged above 60 surveyed in slums had antibodies.
To understand how long antibodies last, the BMC is planning to conduct a longitudinal study to retest those surveyed in the first and second round. “This will give us an idea whether they still have antibodies or not. We plan to retest healthcare workers who had volunteered earlier,” said Dr Daksha Shah, deputy executive health officer, BMC.
Just like the first survey, women showed more sero-prevalence than men in both slums and non-slums. Of 1,405 women tested, 46.2 per cent had antibodies in slums. As opposed to that, of 1,619 men tested in slums, 44.4 per cent had antibodies. In non-slums, of 721 women surveyed 17.6 per cent had antibodies, while of 1,452 men surveyed only 16.7 per cent had antibodies.
“Maybe women develop antibodies faster than men. We don’t know if exposure rate is higher or if immune response is quicker. We need further research,” said Shah.
Professor Ullas Kolthur, from TIFR, said there could be multiple factors that could have contributed to the differences in prevalence in men and women. “It could also be attributed to social behaviour wherein women go out or intermingle more and hence more exposure. Research has shown that the immune response in men and women is different, including T-cell response. Although, the high prevalence in women is unlikely because of a significant difference in the antibody response, it cannot be ruled out. We need to research more,” he said.
The team is also assessing RBD antibodies and neutralizing antibodies. It will be interesting to check how levels of nucleocapsid antibodies and RBD antibodies correlate with virus neutralization.
The study observed that reduction in overall sero-prevalence could be due to how antibodies behave. “The reduction in sero-positivity in slums could be due to a decrease in antibody titers in a significant proportion of the population who were possibly infected very early in the pandemic, unlike in the non-slums,” TIFR observed in the study.
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