What could be driving Mumbai’s COVID surge?

The extent to which new variants are circulating, and reinfections occurring, needs to be investigated urgently

March 30, 2021 12:15 am | Updated 12:59 pm IST

Mumbai has recently seen a sharp rise in COVID-19 cases, recording on average 5,500 new cases every day over the past week. The weekly total is, by far, the highest the city has ever seen. What does the data tell us about this surge?

First, cases have shot up, doubling every week or so. Such speeds were last seen in April 2020. Housing societies were hit first, but the surge rapidly spread to the slums too, where we know that for each infection found, many more are missed. Although the slums are still generating a minority of cases, active infections could well be at the same level as in housing societies, if not higher.

The past week has seen a welcome rise in tests. While daily tests are higher than ever before, testing is struggling to keep pace with the rise in infections. Over the past week, test positivity (the ratio of cases to tests) averaged about 13%, a value not seen since October 2020. If we correct for the fact that rapid tests are less sensitive than RT-PCR tests, this number would be higher.

Not at herd immunity

What makes the current situation remarkable is that it comes when so many people have already had the disease. Seroprevalence surveys provide estimates of how many people have developed antibodies to SARS-CoV-2, the virus which causes COVID-19. Mumbai’s surveys, taken together with case and fatality data, suggest that at least half the population — quite likely more — has been infected at some point.

In theory, people who have already had the disease should act as a buffer, slowing down spread. This assumes, crucially, that after infection they become immune to the disease. The theory is simple: if half the population is immune, then one infected person typically infects only half the number they otherwise would. If a sufficient fraction of the population is immune, outbreaks naturally die out — at this point the population has reached ‘herd immunity’.

Clearly Mumbai is not at herd immunity, but some level of immunity should still slow new outbreaks. And here comes the catch: Mumbai’s current outbreak has grown more rapidly than basic calculations would suggest possible. It is not the absolute numbers, but rather the rate of increase which seems at odds with the theory. So, what could explain the rapid increase?

Possible explanations

First, simple arguments about immunity ignore the crucial fact that people interact with each other in networks, and not ‘randomly’. This means that spread can be uneven: some pockets of the city may have remained relatively unscathed earlier and vulnerable to rapid surges. It was widely expected that if the city returned to normality too quickly, there would be a rise in infections.

Uneven spread and opening up had a clear role in driving the city’s second surge and are likely playing some part in this one. However, in contrast with the second surge, recent ward-level data show quickly rising cases across the city’s wards. Something more is happening this time round.

This brings us to more speculative questions. Could it be that a SARS-CoV-2 variant which is more transmissible – which spreads from person to person more easily – is circulating in the city? In this case, a relatively small fraction of susceptible people could drive rapid growth, helping explain the rapid rise.

Also, could the city be seeing a significant number of reinfections? This might occur if immunity acquired through infection wanes over time or can be bypassed by a so-called ‘immune escape’ variant.

The high prevalence of the so-called ‘U.K. variant’ in Punjab (http://bitly.ws/coMQ), and the presence of a novel variant with a double mutation in Maharashtra (http://bitly.ws/coMU), indicate that we should not ignore the possibility of variants playing a part in Mumbai’s new surge. Hopefully increased genome sequencing will soon shed more light on their role in Mumbai.

Will the death toll rise?

In terms of mortality, Mumbai has already suffered badly. There have been 11,661 recorded COVID-19 deaths in the city so far (as of March 29). But data from the municipal corporation shows an even greater toll: the city saw a huge 24% increase in all-cause mortality in 2020, namely about 22,000 more deaths than expected from the previous five years’ data.

So, it is some relief that daily recorded COVID-19 deaths in the city remain well below values during previous waves. On the other hand, we expect delays between infections rising and deaths rising. On average, daily deaths have risen from around five at the beginning of March to around 13 today, and judging from a sharp and continuing increase in hospitalisations further rises are likely.

The hope is that mitigation and prior immunity, augmented by vaccination targeted at those most at risk of severe disease, could together reduce deaths during this wave. But counting on this when infection is spreading like wildfire would be dangerous.

In summary, it seems likely that multiple factors, including uneven spread and opening up of the city, are playing a part in Mumbai’s latest COVID-19 surge. The extent to which new variants are circulating, and reinfections occurring, needs to be investigated urgently.

It goes without saying that Mumbai-dwellers need to be extra careful at the moment – with an emphasis on masking up and avoiding unnecessary crowds and indoor gatherings. This surge still has a long way to go.

Murad Banaji, Senior Lecturer in Mathematics at the Department of Design Engineering and Maths at Middlesex University, London, has an interest in disease modelling

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