Mumbai's sero-survey showed that 57% of the samples collected from the slums and 17% of the samples collected from the non-slum areas in M-west, R-north and F-north wards of Mumbai have developed antibodies against COVID-19 in its preliminary findings. The Brihanmumbai Municipal Corporation (BMC) collected around 6936 samples from these areas.
The higher prevalence in the slums was attributed to the high population density in the slums and even the sharing of local facilities. Effective containment and social distancing measures took the credit for the low prevalence in the non-slums. The survey collected 100% samples from the slums and around 70% samples from the non-slum areas to measure the presence of SARS-CoV-2 in the population.
This survey which was commissioned by the BMC, NITI Aayog, Tata Institute of Fundamental Research was also carried in collaboration with Kasturba Molecular Diagnostic Laboratory, Translational Health Science and Technology Institute, A.T.E Chandra Foundation, and IDFC Institution.
The surveillance that was conducted in the first half of July shows that the prevalence of the virus was "marginally higher" in women than in men. The ward-wise and age-wise stratification, however, was comparable in the wards.
BOOM spoke to Dr. Daksha Shah, Deputy Executive Officer, Health, BMC for understanding the implications of the study. "Complete implications will be decided after the second round that will be conducted in the same areas to validate the findings. As policy measures, we can say that two level protection has been achieved in the slums. The second round will help in further validating these findings."
Findings of the Survey
A sero-survey is carried out to understand the prevalence of a disease in a population. Blood tests find the presence of antibodies against a certain pathogen.
Through this survey, around 57% of the participants from the slums and 17% of the participants from the non-slums showed IgG antibodies against SARS-CoV-2. All these people did not show symptoms but were infected. According to the officials, the infection fatality rate in these wards is around 0.05 to 0.10%. Infection fatality rate is the number of deaths divided by the total number of infections. These include people that were infected but did not show symptoms.
Asymptomatic infections are surmised to be present in a high proportion of the population. High population density and sharing of facilities are the leading reasons for the high prevalence in the slums, the study believes. For the non-slums, the low prevalence is predicted to be due to the containment measures of social distancing and mask wearing.
These findings will assist the civic, scientific, and national bodies formulate policies to understand the prevalence of herd immunity. So far, the levels to attain herd immunity against COVID-19 are unclear, but the interpretation of the findings suggest that if this immunity continues to exist, it could be achieved in the slums.
This data needs to be further researched for neutralising antibodies- whether these antibodies protect against further infection and also help in identifying risk factors for SARS-CoV-2 infection, opines the BMC.
These three wards were chosen due to the growth of cases in the month of June. This study was a part of a nationwide sero-survey conducted by the Niti Aayog. In Delhi, a sero-survey found around 23% of the population with antibodies while the first round of the ICMR found a sero-prevalence of 0.73% in 83 districts.