Mumbai’s daily Covid-19 cases are climbing sharply. Over the past week, cases have averaged almost 1,600 per day – triple the number one month ago. Meanwhile, a small but increasing fraction of the city has had at least one dose of a Covid-19 vaccine, probably around 4% to date.

How bad is the city’s third Covid-19 surge? And where is it likely to go next? Could vaccination halt the wave?

These are difficult questions, but we now understand a lot more about Mumbai’s epidemic than one year ago. This added knowledge means that we can put the numbers into perspective.

How fast is disease spreading?

We know from serosurvey data that recorded cases represent only a fraction of total infections. These surveys involve sampling populations to estimate “seroprevalence”, that is, to find how many have antibodies to SARS-CoV-2, the virus responsible for Covid-19. This fraction of infections recorded as cases is likely to be much lower in slums than in nonslum areas. We can try to estimate daily infections in the city with the help of previous data, but we need, first, to look at testing.

Currently, Covid-19 testing in Mumbai is at its highest level ever, averaging about 20,000 daily tests, up from about 15,000 a month ago. About 30% of these are rapid antigen tests. This fraction has been quite steady.

Test positivity – taken as the ratio of confirmed cases to total tests – has doubled to 7% in the last month. The rise in testing has not been fast enough to keep pace with the rise in infections, and detection of infections is almost certainly going down.

The rise in test positivity is a worry, but its value is still relatively low compared to previous highs. During the first wave (April-May, 2020) test positivity averaged over 20%, sometimes crossing 30%, while during the second wave (August-October, 2020), test positivity averaged around 15%.

Extrapolating from existing data suggests that close to 10% of infections are currently being picked up in testing. If correct, this implies about 17,000 daily infections over the past week, as compared to estimated peaks of over 50,000 during the second wave and over 100,000 during the first.

So, spread during this wave has not reached the speeds seen in previous waves – so far. But the rise is rapid and a few weeks from now the picture could be very different.

A mural in Mumbai. Credit: Indranil Mukherjee/AFP

A nonslum wave, spreading to the slums

The wave is not limited to a few areas of the city. On the contrary, most wards in the geographical centre of the city are seeing rapid growth in case numbers, with many in the north and south not far behind. One lesson from Mumbai’s past data is that the main divide in the city is between slum and nonslum areas. So, is the spread occurring in the slums or in nonslum areas?

The municipal corporation does not separately report cases from slum and nonslum areas. However, we can estimate this breakdown using ward-wise data. The estimates tally with occasional claims by the municipal corporation: for example, it reportedly claimed that 90% of cases in the past two months have been from housing societies. This compares to ward-based estimates of around 84% of cases from nonslum areas.

The ward-wise data shows very clearly that the current surge began in nonslum areas. During two weeks around the middle of February estimated cases in nonslum areas doubled, while the increase in slum cases was relatively minor.

But then slum cases started to rise more sharply, more than doubling in the last three weeks. There has been a sharp rise in “active containment zones” in slums and chawls across the city: since February 25, these have shot up from 13 to 31. There have also been news reports of a rise in cases in Dharavi. Slum cases currently still account for only about 20% of the total. But we know that detection in the slums is weaker, and the slums could actually be generating 40% of the city’s daily new infections.

Fatalities – still low, but for how long?

The toll of the pandemic on Mumbai so far has been huge. The year 2020 saw more than 11,000 recorded Covid-19 deaths, and nearly double this number of excess deaths according to data from Mumbai’s municipal corporation. This amounts to a 24% rise in overall mortality over the previous five year average, or roughly 60 extra deaths a day throughout the year.

With these huge numbers looming over the city, it is a relief that the latest surge has not, so far, translated into a rise in daily fatalities. During August to October 2020, when the second wave was occurring, there were an average of 40 recorded Covid-19 deaths every day in the city. The current figure is about four.

But could a rise be on the cards – after all, we generally expect some delay between a rise in cases and a rise in deaths?

Quite probably. Covid-19 hospitalisations have risen. In the last month there has been a 40% rise in ventilator and ICU bed use, while the number of O2 beds in use has more than doubled. Total hospitalisations remain well below previous peak values, but the rises are concerning. This is likely to translate into an increase in deaths.

Optimistically, we might hope that fatalities will be limited during this wave – if, for example, a higher proportion of cases are mild at the moment. This could be the case if more recent infections are among young people, or a sizeable fraction are reinfections. Vaccination of vulnerable groups will reduce the death toll.

Mumbai residents at a vaccination centre. Credit: Reuters/Francis Mascarenhas

Are reinfections occurring?

There is not yet very strong or direct evidence that reinfections are a major part of the story, but we can’t be sure. Most new infections still appear to be from nonslum areas where prior infection has been lower. But the rise has been fastest in the slums recently, and if a major slum surge develops, this would suggest an important role for reinfections.

If reinfections are occurring, this could simply be because a lot of infections occurred quite a long time ago, and immunity is waning. But another possibility involves new variants – mutant strains of the virus. Could these be playing a part?

If new variants are circulating widely in Mumbai, then we would like to know:

  • Are they more transmissible – i.e., do they spread more easily?
  • Are they more deadly – i.e., do they lead to more severe disease and death?
  • Are people previously infected more vulnerable to reinfection by them?
  • How effective will current vaccines be against them?

    The third question could be the most important given the large number of Mumbai dwellers previously infected. “Immune escape” refers to the ability of a variant to evade immune responses developed after previous infection. If a variant capable of immune escape becomes widespread, this could dramatically worsen Mumbai’s epidemic.

    Unfortunately, we don’t know much about the levels of new variants circulating in Mumbai. A number of mutant strains have been found circulating in Maharashtra. But there is only partial data on which of these are more transmissible, more deadly, or liable to evade immune responses. How these variants are affecting the progress of the epidemic in the state is not yet clear.

Some key lessons

Mumbai is seeing a rapid surge in Covid-19 cases, which have reached levels not seen since October. The current wave began in nonslum areas, but has spread to the slums. Although official data has not yet shown an increase in deaths, a rise in hospitalisations is occurring and this could soon translate into more fatalities.

Although it is not yet clear what part reinfections are playing, the belief that high levels of prior infection are protecting the city could be dangerous. The city authorities must explain frankly to people that being infected once is no guarantee against getting infected again. If the slum surge becomes large this will strongly suggest that reinfections are a serious problem.

Mumbai needs to learn from cities like Manaus in Brazil which saw a major resurgence despite high levels of infection. The response must include a step-up in testing and contact tracing, especially in the slums where detection is weakest. More genome sequencing is needed to find whether there are circulating variants able to evade the immune defences of people previously infected. In fact, genome sequencing across the country needs to be ramped up with some urgency.

Could vaccination halt the current wave? Not right away. Less than 0.5% of Mumbai’s population are being vaccinated per day and it takes time for protection to develop after vaccination. But the focus on vaccinating the elderly and vulnerable will certainly reduce hospitalisations and deaths.

It is crucial that vaccines reach slum populations and not just more affluent sections of the city. The municipal corporation is making welcome moves in this direction with door to door visits and free transport to and from vaccine centres.

Mumbai’s current Covid-19 wave is very much an unfolding story with lots of uncertainty and many possible futures. The epidemic is far from over.

Murad Banaji is a mathematician with an interest in disease modelling.

All data used is taken from Mumbai’s detailed daily Covid-19 dashboards. The methods used for estimation were described broadly in a piece on the city’s second wave here and linked technical documentation.