A total of 57% individuals from slums and 16% from other residential areas in three wards were exposed to Sars-CoV-2 and recovered silently owing to the antibodies produced, the sero survey conducted by the Brihanmumbai Municipal Corporation, as part of the national study to understand the spread, has found. Individuals in both these groups were asymptomatic.
“Overall, we have observed that 40% of those surveyed, in both slums and residential areas, were infected with Covid-19, but recovered. None of those tested for sero survey had to undergo RT-PCR test, as they were asymptomatic,” said Suresh Kakani, additional commissioner, BMC. “The prevalence of the virus in slums is high due to densely populated areas. In non-slums (residential areas), better social distancing and maintenance of hygiene helped stop the spread to a large number of people,” said Kakani.
On July 3, the civic body, in collaboration with NITI-Aayog and Tata Institute of Fundamental Research, started sero surveillance, in which a group of individuals undergo blood tests to detect the presence of Immunoglobulin-G (IgG) antibodies. IgG antibodies are produced by the body’s immune system on being exposed to a virus. A sero survey helps identify individuals who were previously infected with the virus and have now recovered. Although the estimated target was 10,000 blood samples, the ongoing monsoon restricted the collection process to 6,936 samples, said a health officer from BMC. The civic body has also run an antibody test on 1,000 health workers, results of which are pending.
BMC officials believe the infection fatality rate (IFR) is likely to be low, between 0.05% and 0.10%. The civic body did not divulge the estimated prevalence of infection. IFR is the ratio of the number of deaths that occur from a disease in the total number of infected people. It is lower than CFR, which considers only confirmed cases (see box). “Results of the sero surveillance are promising because it shows that a large number of people have recovered from the virus with the help of their own immunity,” said Dr Om Srivastava, epidemiologist and member of the state’s Covid task force.
The results will be re-evaluated later to learn about development of herd immunity, when a large portion of a community (the herd) becomes immune to a disease, controlling its transmission and spread.
“Although it is still unclear what level of prevalence leads to herd immunity, our 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,” read the interpretations of BMC.
Dr Srivastava overruled the possibility, as the antibodies produced in the body are not permanent. “Antibodies stay in the body from four weeks to a few months depending on the infection. Depending on single data set, we can’t say that we have reached herd immunity. It is not a representative sample to come to any scientific conclusion,” he said.
While 60% patients diagnosed with Covid-19 have been male, the survey revealed a greater number of women than men were exposed to the virus and recovered. The civic body did not share data on the proportion of male-female samples.
“It is a known fact that women have stronger immunity than men. This helps them to fight the virus better. Most women don’t even develop symptoms to get identified with the virus,” said Dr Daksha Shah, deputy health officer, BMC.
BMC will conduct another sero surveillance next month in the same three wards for a comparative study to gauge the spread of Covid-19 infection.