A sero-prevalence study conducted by Mumbai's civic body, the Brihanmumbai Municipal Corporation, on 6,936 people from three municipal wards found that 57 percent participants in slums had been exposed to and developed antibodies against SARS-CoV-2 virus. This was against just 16 percent of those tested in non-slum areas and residential societies.
High density of population, lack of social distancing and common utilities like shared toilets have been inferred as the biggest reason for such high exposure and sero-prevalence in slums versus housing societies.
The three wards circled areas in Dahisar, Matunga and Chembur. Prevalence of antibodies was marginally higher in women; however, age wise the prevalence was comparable across the three wards.
BMC's sero-survey also shows that a high proportion of asymptomatic infection is likely to have spread.
Sero-prevalence study checked for antibodies against Sars-Cov2, indicating past exposure to the virus. The BMC survey was done in the first two weeks of July. Mumbai had reported its first case on March 3 and till July 28 had reported 1,10,882 clinically confirmed positive cases.
The sero-prevalence data is significant as it indicates a general level of immunity against the virus being built in the population being surveyed. BMC in its statement said, "Although it is still unclear what level of prevalence leads to herd immunity, 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."
However, the study was done in just three wards in Mumbai and experts say cannot be representative of the whole city. BMC will be conducting a second round of sero-survey in other wards.
The sero-prevalence or virus spread data also alters the fatality graph for the city. BMC says the infection fatality rate taking into account current prevalence and reported COVID-19 deaths is likely to be very low at 0.05 - 0.10 percent.
Infection fatality rate is different from cases fatality rate, which records deaths as a percentage of all confirmed positive cases, which have been clinically diagnosed through RT-PCR/antigen test.
Infection fatality rate is a boarder indicator of fatalities as against all infections, including clinically confirmed cases and those who may have been exposed or infected with the virus, developed antibodies and recovered on their own.
Public health experts say infection fatality rate is a more accurate indicator to study disease spread and its casualties. BMC said, "Together with relatively low prevalence in the non-slums it suggests that social distancing and related precautions such as wearing masks are effective in slowing the infection spread and should continue as a new normal in all sections of the society independent of prevalence."
Experts say while the civic bodies containment measures and social distance practices have kept the non-slum areas low, it also indicated that these regions are still largely vulnerable to the virus.
The BMC conducted the sero-survey with the Tata Institute of Fundamental Research and Niti Aayog. Slums and non-slums were equally represented in the survey group. Abbott’s chemiluminescence assay (CLIA) tests were used for the antibody studies.
Last week, Delhi had released its sero-survey results showing that 23.6 percent of the people tested had exposure to the novel coronavirus. A recent report from Ahmedabad Municipal Corporation reported the largest antibody survey where 30,054 people were tested and 17.61 percent were found to be positive.
The population sero-surveys are also useful in indicating if the population has achieved or is approaching herd immunity against the disease. For Sars-Cov2, many public health experts have quoted 50-60 percent infection spread could be an acceptable bench mark for achieving herd immunity.
However, the current sets of antibody test kits have their own weaknesses of reporting high false positives. K Srinath Reddy of Public Health Foundation of India had opined in a recent CNCB TV18 column that this must be kept in mind while drawing inferences from any sero-survey. "False positive test results can inflate the estimated infection rate in the general population, much more than they do in a high prevalence hospital population.”
BMC has indicated that the ongoing study will provide information both on presence of neutralising antibodies and on risk factors on SARS-CoV2 infection. Planned repeat surveys will provide information about infection spread in both slums and non-slums, and could inform about herd immunity.