A serological survey conducted by the Brihanmumbai Municipal Corporation, Mumbai’s civic body, has shown that 57% of people in slums and 16% in residential areas may have been exposed to the coronavirus. These individuals were asymptomatic and may have quietly recovered from Covid-19, given the presence of antibodies in their bodies.
“Overall, we have observed that 40% of those surveyed, in both slums and residential areas, were infected with Covid-19, but recovered,” BMC Additional Commissioner Suresh Kakani said, according to the Hindustan Times. “None of those tested for sero survey had to undergo RT-PCR test, as they were asymptomatic.” He said that the prevalence of the virus was high in slums because of crowded dwellings, and much lower in residential areas due to physical distancing and better maintenance of hygiene.
According to the US’ Centers for Disease Control and Prevention, serology tests are “those that look for antibodies in blood”. “If antibodies are found, that means there has been a previous infection,” it says. “However, there is a chance a positive result means that you have antibodies from an infection with a virus from the same family of viruses (called coronaviruses), such as the one that causes the common cold.”
Serological surveys reveal how many people may have been infected with the novel coronavirus in an area but they do not show how many people are immune to the virus. It still isn’t clear how long antibodies last in infected persons and how many antibodies are needed to protect a person from reinfection.
The serological surveillance began on June 3, in collaboration with NITI Aayog and Tata Institute of Fundamental Research. The study period was the first 12 to 14 days of July. The survey involved a group of people undergoing blood tests to detect the presence of Immunoglobulin-G antibodies. The body’s immune system produces IgG antibodies on being exposed to a virus.
The target sample for the survey was 10,000 individuals, but only 6,936 could be collected due to the ongoing monsoon season, the BMC said. Testing was carried out in three wards – R-North, M-West and F-North. The civic authority has also run an antibody test on 1,000 health workers, results of which are pending. The BMC will conduct sero-surveys for other wards in the coming months.
The Infection Fatality Rate, or IFR, is very low in Mumbai at 0.05% to 0.10%. IFR is the ratio of the number of deaths that occur from a disease in the total number of infected people, whereas the Case Fatality Rate, or CFR, measures the death rate only for confirmed cases.
“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,” Dr Om Srivastava, an epidemiologist who is part of Maharashtra’s Covid-19 task force said.
The BMC will evaluate the results to understand the possible prevalence of herd immunity. “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,” the civic body said in a statement.
Mumbai on Tuesday reported just 717 cases, the lowest figure in three months, while 2,467 patients were discharged, reducing the number of active cases to 20,251, according to the BMC.
A similar serological survey conducted in Delhi had shown that 23.8% of the respondents carried antibodies.
In Mumbai, around 60% of the patients diagnosed with the coronavirus were male. However, the sero-survey showed that a higher number of women were exposed to the contagion. “It is a known fact that women have stronger immunity than men,” BMC Deputy Health Officer Dr Daksha Shah said. “This helps them to fight the virus better. Most women don’t even develop symptoms to get identified with the virus.”
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