COVID-19 second wave: Why no one saw this coming

COVID-19 numbers in Mumbai

Photo: Meghana Sastry, Creative Commons

With Mumbai recording 7367 Covid-19 cases and 75 deaths as on April 22, the second wave of the pandemic has overwhelmed the maximum city. 

Mumbai has reported more than 1.38 lakh cases in April alone so far whereas in  September 2020, during the ‘first wave’, the city had reported 57,755 cases. 

Until April 23, Mumbai has conducted 51.22 lakh tests and has reported 11.75% test positivity rate till date. The city’s healthcare infrastructure is under strain with only 7139 COVID beds vacant as on April 22. Of these, only 42 are ICU beds and only 21 are ventilator beds that are available. 

In Mumbai, by April 22, 8.31 lakh people above 60 years of age had taken at least one or both doses of a vaccine. The corresponding figure for those above 45 years is 7.52 lakh. 

Maharashtra has reported 40.94 lakh cases and 62,479 deaths as on April 22. The state has a case fatality rate (CFR) of 1.53%. 

Dr Murad Banaji, a mathematician who has been looking at Mumbai’s Covid data and who is also a senior lecturer at Middlesex University, London, spoke to Citizen Matters about his calculations for the ‘second wave’.  

Dr Murad Banaji

Can you tell us how it got this bad so fast in India in general and Maharashtra, in particular? If the government did not see this coming, did mathematical models also not predict the second wave of COVID-19?

I cannot speak for the government but my models did not see this coming. Many people, including myself, had warned of a second wave nationally. In terms of Mumbai, which I follow much more closely, I did not see such a severe wave coming. If all interventions (such as restrictions on movement) were stopped at a given point, the models suggested a new surge would occur, but not of the speed or scale that we are seeing at the moment. This kind of massive surge was quite hard to predict based on the data I was putting into the models.

Mumbai is seeing a high test positivity rate right now. Does that mean that we are testing more and better now or does it simply mean infection rates are that high?

Generally high test positivity means that you are probably missing a lot of infections. But interpreting test positivity is not easy – it depends on the testing strategy. One way to think about it is as follows: If random people on the streets are tested, you won’t find that many cases. Test numbers will be high, cases will not be so many, and the positivity rate will be lower. But if the majority of testing is of high risk groups like people showing symptoms or close contacts of people who tested positive, positivity is higher. So you have to be a bit aware of what strategy is being used for testing. In general, if you see positivity go up, like in Mumbai recently, it means infection spread too fast for testing to keep up. To my mind, it means a lot of infections are being missed. So for every case, there are many, many more infections getting missed. 

Exactly how many more are being missed and why?

If detection levels (percentage of infections detected in testing) are the same as first wave, then the great majority of infections are being missed. We know from the city’s first serosurvey that around 1% of infections in the slums and around 6% in housing societies were detected. This means that for every one infection in the slums found there were 100 more which were missed. How are so many infections going undetected? It is probably a mixture of factors including the young population in the slums – those who are asymptomatic or with mild disease remain under the radar… But it is more than just the young slum population: the chances of getting tested in the slums were small in the early part of last year. Even in a slum like Dharavi which was watched closely by India and the world, testing was poor.

Read more: Explainer: how to get vaccinated in Mumbai

Since we mentioned Dharavi, the city administration’s efforts were lauded for containing the spread in Dharavi in 2020 but it is also seeing a rise in cases now…

I have strong feelings about the Dharavi story. I don’t doubt or deny the effort that went into containment – a lot of effort and work went in there. What we need to ask is how effective were those measures? If you look at the spread of infection, Dharavi is not a success story. The first national sero-survey had covered Dharavi and had already found around 36% sero-prevalence there in May, 2020. Based on the city’s later surveys, at the end of the first wave, upto 60% of residents could have been infected. Even though much of this infection may have been mild or asymptomatic, if you look at Dharavi’s CFR, it was around 10% while CFR for Mumbai as a whole was around half of that. The same pattern is seen across the city – much higher CFR in the slums. What does that actually mean? High CFR in slums implies that even people who fell quite ill were not picked up by the system. If you were picking up most symptomatic people, then 10% shouldn’t die. Many stories from the slums suggested that disease was being picked up late and by the time the patient would be taken to a hospital, they were likely critical. During that period, the chances for a middle class person to get a hospital bed were not good – but for a slum-dweller they were that much worse.

Coming back to missed infections, how can an overburdened system in an over-populated city improve on this count?

I think detection has improved even though test positivity is very high. Certainly daily tests in the city are much higher now than last year. Of course, there is still room to scale up testing. Some of the current testing involves rapid tests. These risk missing some infections, but are easier to scale up, so I would support a major scaling up of rapid testing in the city. Also, it seems that rapid tests tend to pick up cases where there is a higher viral load – more current infections. Since a lot of the rapid testing seems to be somewhat random – e.g., at malls (when they were open), or at railway stations – the positivity of rapid tests gives an indication of how much active infection there is in the city. So, for example, 10% positivity for these tests could indicate that around 10% of the city is currently actively infected. This is also the figure I get by looking at other data. But of course there is a lot of uncertainty – to be more confident we’d need to do proper random testing. 

Could you tell us how you arrived at this estimate of 10% population being actively infected in Mumbai right now?

If we look at earlier periods of Mumbai’s epidemic and the results of the city’s serosurveys we get some idea of how many infections get found in testing when test positivity is at current levels. Applying these estimates to the latest data indicates that at peak, 10% or more of the city could have been actively infected – if active infection is taken to last about one week. This is also consistent with the test positivity from rapid tests which has been hovering between 10 and 15%. 

So is the government’s decision of a lockdown-like situation justified? It would prevent these infected people from mingling with the rest.

But I don’t know what lockdown means? I think you have to look at the first wave and say, some of the measures worked and some measures failed. You cannot lock up people in slums. There is a real danger that you accelerate the spread of disease by confining people in this way – that the slums become “incubators” of disease. Right now, people who do not need to move around should not be moving around. If you can work from home, then that’s the right thing to do. Gatherings should be avoided at all costs and masks should be worn. Obviously people need to be supported financially if they cannot work. But the notion of lockdown has acquired a very strong negative connotation for the people of Mumbai. Instead of talking about lockdown, it is better to list out the measures, try to focus on their scientific basis, and explain risk to people. For example, given what we know of COVID, it is not clear if there is any good basis for shutting outdoor spaces like parks and beaches, or limiting the timings of shops – both can inadvertently cause more crowding and increase risk. 

Photo: Sarabjit Singh (Tribune India), Wikimedia Commons

Despite the high number of cases, there has been talk since last year about India’s fewer deaths as compared to other countries. Even in this second wave, number of deaths is lesser than the worst we have seen. Could you shed some light on that?

The whole narrative about India having low mortality is driven by weak surveillance, low-quality data, and weak analysis.  Unless we eventually get excess mortality data from across the country, it will be very hard to know the toll of the pandemic. In Mumbai we do have sanity checks. It has better surveillance, and relatively good access to healthcare and testing compared to many parts of the country. You are less likely to have Covid deaths going uncounted. And yet, excess mortality in Mumbai was double the total number of reported Covid deaths in 2020. That difference is likely to be much higher in some parts of the country. If we believe India has done better in terms of Covid mortality, then we should ask: why has Maharashtra – in official figures – done worse than, say, Uttar Pradesh or Gujarat? It is probably largely down to  better recording of Covid deaths. There may be other factors, of course, for why COVID spreads particularly fast and widely in Maharashtra – including its level of urbanisation. We can hope that the fatality rate in Mumbai will be lower this time for two reasons: so many people have been infected in the past; and an increasing number of vulnerable people have now had one dose of a vaccine. My median estimate is that during 2020, 60% people were infected  in Mumbai. It doesn’t necessarily prevent you from being re-infected but we can hope that this should, mostly, protect you from severe disease. That would be one explanation for why deaths might be fewer this time around. Plus we are now vaccinating a small but important part of the population, that is, senior citizens and people with comorbidities, who are most vulnerable to severe disease. If half the 60+ have been even partially vaccinated, that should really bring down the death toll this time. So, hopefully, infection fatality rate will be lower than last year. But we must not forget that Covid keeps throwing up new factors like variants, and some of these can be more lethal. In addition, if health infrastructure gets overwhelmed there are avoidable deaths of both COVID patients and others.

The recovery rate for Maharashtra also seems to be good…

Recovery rate is a useless statistic and should be junked. In a long running epidemic, there are a lot of recovered people. It doesn’t tell you anything about the present reality. If you want to know how bad things are, look at deaths and look at hospitalisations – look at current data. 

When can Mumbaikars expect some relief from this second wave? 

If the current interventions are successful, we should see a winding down about now. We are seeing hints of a slowdown at the moment. The number of daily cases in Mumbai has been below 9000 for a few days now. Test positivity has also come down a little. At least it’s a clear break from the pattern of steady rise before that. Now the question is, do daily cases and test positivity continue to fall or keep hovering? I am hoping they will keep falling now. But I think the situation will improve slowly – not as fast as we would like. If this wave follows the pattern of the first one, there could be a drop in  infections in the slums because of herd immunity effects, and a drop in non-slum areas because of effective measures. But we should be careful about extrapolating from previous waves when there are new factors in play we don’t understand fully such as variants. On this subject, Mumbai urgently needs to see more genome sequencing so we can find out what variants of the virus are circulating in the city and driving the current surge.

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