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How the Mumbai COVID-19 Sero-Survey Results Compare to What We Already Know

How the Mumbai COVID-19 Sero-Survey Results Compare to What We Already Know

A general view of slums next to high-rise buildings in Mumbai, June 27, 2020. Photo: Reuters/Francis Mascarenhas/Files.

Mumbai: COVID-19’s trajectory in Mumbai has taken a turn for the better, with the number of daily cases and deaths getting on a declining trend, and WHO praising the city’s success in getting the Dharavi cluster under control. The results of the recent seroprevalence survey in Mumbai sheds further light into this overall picture, giving us a better understanding of the disease’s dynamics.

The seroprevalence survey, or seroprevalence survey, looked at the levels of immunoglobulin G (IgG) antibodies in 6,936 Mumbaikars across slum and non-slum areas in three wards (including Dharavi). The presence of IgG indicates that the person has recovered from a COVID-19 infection in the past. According to the survey, 57% of those in slums and 16% of those in non-slum areas had these antibodies.

What the survey has demonstrated is that COVID-19 is more widespread in Mumbai than was believed. Approximately 42% of Mumbaikars live in slums. If we assume a similar distribution in other parts of the city and extrapolate the data across the board, the overall seroprevalence of COVID-19 in Mumbai becomes around one-third (33%). This is much higher than the seroprevalence reported for Delhi (23%), New York (23%) or London (17.5%), and is in fact probably the highest reported across the world.

One must also remember that not all individuals who had COVID-19 may have developed antibodies to it, and that the antibody test is not 100% sensitive – i.e., there will be some patients with antibodies present but which the test will miss. In other words, the actual prevalence may even be slightly higher than what the study reported.

Finally, let’s bear in mind that the survey does have some limitations, and that we may need to update our interpretation and understanding once the paper underlying the survey becomes available.


Taken at face value, the survey’s results reinforce certain key aspects of the pandemic. For one, they clear emphasise the extremely infectious nature of the virus, demonstrating that it is virtually impossible to contain its spread in densely populated regions even with a strict lockdown.

The basic reproductive ratio of the virus, which is the number of persons infected by one positive patient, is clearly much higher in the slums than officials had surmised. This has an important epidemiological implication: the bigger this number, the higher the percentage of population that needs to be infected for herd immunity to be achieved.

The Brihanmumbai Municipal Corporation (BMC) has hypothesised in a press note that the population density and the shared common facilities like toilets and water points could have catalysed the virus’s spread. Given the positions of WHO and the US Centers for Disease Control remain that fomites – common touch surfaces – may not be a major source of disease transmission, the population density and lack of physical distancing are likely more important factors.

The survey also showed that a vast majority of the infections remain asymptomatic or mildly symptomatic, with the infection fatality rate being as low as 0.05-0.1%, according to BMC. However, there is also a corollary: that disease transmission from asymptomatic patients might be quite high.

Finally, with over half the slum population having had the infection, what is also clear is lockdowns can’t be used to contain the virus in densely populated areas. Instead, in these areas, the containment strategy should focus on universal masking – which is still wanting – along with better ICU bed availability, which is crucial to reduce mortality.

Finally, the answers to two questions that have lingered in many of our minds – “Is Mumbai heading towards herd immunity?” and “Is herd immunity something densely populated Indian cities can attempt?” – is a full-throated ‘no’. The international scientific consensus is that it will take a community prevalence of 60-70% for herd immunity to develop. Mumbai as a whole remains at about 33%. And it did take 4-5 months for Mumbai to reach this stage, with a lot of mortality and morbidity in the interim.

Whether the antibodies are protective or not and how long-lasting the immunity will be is still not clear. Finally, given that the reproductive ratio for the novel coronavirus in Mumbai appears to be higher than what was presumed, the fraction of the population that needs to be infected to achieve herd immunity is also higher, and therefore untenable.

In May, the Indian Council of Medical Research (ICMR) conducted a seroprevalence survey across 80+ districts, and found a seroprevalence of 15-30% in hotspot areas (although the full results are yet to be published). The Mumbai-specific details are not available but one could assume that health authorities have access to them. Although the ICMR survey had a much smaller Mumbai-specific sample size compared to the current survey, the current survey results demonstrate that the infection has continued to spread rapidly in the interim.

The BMC is planning to conduct a follow-up survey in the same three wards but with a different set of participants from August 10 to validate the findings. The cumulative set of findings could provide a good understanding of the temporal profile of infection rates in these areas, i.e. the speed with which the infection spreading.

If the follow-up survey demonstrates a relatively unchanged prevalence in the slums, then it might indeed indicate that herd immunity has been achieved. On the other hand, if the prevalence has increased further, then the results would indicate that 57% prevalence is not sufficient to attain herd immunity.

Officials have since also decided to conduct a city-wide seroprevalence survey. The combined data from these two surveys could help decide the further course of action to more safely ease restrictions in the city, and perhaps even provide a template for the rest of the country to follow, given Mumbai is ahead of the curve right now.

For now, the foremost thing to emphasise is that Mumbai, particularly its slums, can’t get complacent. There are many factors that can alter the balance in slum areas, including returning migrant labourers. However, the results do indicate that people living in slum areas and nearby can cautiously press the restart button in their lives sooner than the rest, while continuing to adhere to basic precautions. The authorities could consider proceeding to the next stages of unlocking – including schools and markets – earlier in the slums, creating local micro-bubbles in a sense with a view to alleviating their economic and educational prospects.

The roadmap for further unlocking will become clearer after the full details of the current survey are published, as well as after the follow-up survey concludes.

Dr Akshay Baheti is an assistant professor at a hospital in Mumbai. The views expressed here are the author’s own.

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