Sero survey points to possibility of herd immunity in Mumbai slums

A door-to-door survey at Ambujwadi slum of Malvani at Malad in Mumbai on July 9.

The findings of a recent serological survey in Mumbai show that the city may have unwittingly reached a level of herd immunity. It shows 57 per cent of the people in the slums and 16 per cent in its residential areas have been exposed to the novel coronavirus, thereby developing the antibodies to the SARs-CoV2 infection. Experts believe this could perhaps be the highest population immunity worldwide.

The survey, commissioned by NITI Aayog, was conducted by the Municipal Corporation of Greater Mumbai (MCGM) and the Tata Institute of Fundamental Research in the first two weeks of July. It covered 6,936 samples from a general population located in three wards, which included slums and non-slums. The participants were recruited on a voluntary basis. Blood tests were done to detect the presence in them of Immunoglobin-G antibodies, which the body’s immune system produces after being exposed to a virus

The survey reveals another positive sign: that Mumbai’s infection fatality rate (IFR), at 0.05 per cent to 0.10 per cent, is very low. The IFR is the ratio of the number of deaths against the total number of infected people. The IFR was calculated on the current prevalence (estimated in the study) and the MCGM records on reported deaths. Among the survey’s other key findings is that the prevalence of the antibodies was high in women.

Interpreting the data, the study team suggests that asymptomatic infections are likely to be a higher proportion of all the infections. The higher prevalence of infection in slums could be due to population density and shared common facilities such as toilets or water points. The lower prevalence of infection in non-slum areas could be the result of better social distancing and access to better hygiene.

However, it may be too early to rejoice. Serological survey reveals how many people have been infected by the virus in a particular area. It does not indicate the immunity level or whether the infected person is immune at all. Scientists are still trying to understand how many antibodies are required for immunity and how long the antibodies will survive in the host.

An official statement issued by the MCGM says: “This study aimed to estimate sero-prevalence in the population based on random sampling methodology from age- and gender-stratified samples from the general population and health-care workers, and at two time points to infer epidemic spread.”

On the results, the MCGM says: “These results will be valuable to learn more about 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 municipal corporation plans to repeat the survey to understand the spread of the virus and gather information on herd immunity.

Even though the findings are of a survey involving a tiny cross section of the city, experts say this perhaps could explain the drop in infection numbers in slum areas, which were expected to be hotbeds of infection.

While the sero-survey results bring a ray of hope to a city that has been struggling to get back on its feet, a few front-line workers in the fight against COVID point out that the sampling is very small and is not indicative of the level of infection in the large population of Mumbai. According to Census 2011, Mumbai has a population of 1.8 crore. Another flaw, says a doctor, is that the age cohort is not explained in the sampling. This is significant as SARs-CoV2 has proved to be fatal for aged people who have other illnesses.