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By now, we know that COVID-19 vaccines save lives. There is plenty of evidence for this from vaccine trials, and from countries such as the UK and Israel which have seen rapid vaccine roll-outs.
In India, vaccination has been slow – a modest 8-9 percent of the population has received at least one vaccine dose. But the drive has, rightly, been focussed on the elderly and others most vulnerable to severe disease. This should have a clear effect when it comes to reducing hospitalisations and deaths. Hard data to confirm this is limited, so far.
One concern is that we don’t know how the vaccines will perform against some variants of SARS-CoV-2, the virus which causes COVID-19. Limited genome sequencing data indicates that in different parts of the country different variants are dominant. Could it be that some of these variants can cause severe disease even in the vaccinated population?
To measure vaccine effectiveness outside of a trial, we would, ideally, track disease and fatalities in the vaccinated and unvaccinated populations, and after matching for age and other risk factors show that vaccinated populations are seeing less disease and fewer deaths. But it is not clear to what extent this kind of monitoring is being done.
Even without direct evidence, we can still look for circumstantial evidence to support a vaccine effect. If, say, we see a sharp drop in fatalities in a population where vaccination has been high, this would be a hopeful sign. But we run into two problems:
Despite these problems, we can check, locally, if there are fewer deaths than “expected” in populations with high vaccine coverage. Let’s try this in Mumbai.
During March, daily cases in Mumbai grew fast, shattering all previous records by mid-March 2021 before peaking at roughly 10,000 around 7 April. Although cases stabilised and even fell a little after this, recorded deaths have risen through April so far, averaging around 61 per day in the week ending 24 April.
Hidden in Mumbai’s data is a very striking trend. During April, the over-60s – let’s say the “elderly”, for short – have formed a declining fraction of the city’s total COVID-19 fatalities.
If this had continued, then in the week up to 24 April, there would have been 300 more COVID deaths in the elderly than observed. We see a strong divergence between total observed deaths, and expected deaths based on trends in the under-60s. Fatalities in the city today are around 44 percent lower than expected.
The huge slum surge during April-June 2020 coincided with a sharp increase in younger people dying.
We can confirm this by looking at the case fatality rate (CFR). We find that a rise in cases at a given time generally leads to a rise in recorded deaths around 18 days later. By taking the ratio of deaths at a given moment to cases 18 days earlier, we can calculate Mumbai’s delayed CFR, and find a drop of around 33 percent during April.
The drop, alongside the age-shift, tells us that the dominant effect is fewer deaths in the elderly, and not more in younger groups.
In fact, the 33 percent drop in CFR during April could underestimate the true fall in the COVID-19 fatality rate: the rapid rise in cases and test positivity during late March and early April should lead to a rise, not a fall, in the delayed CFR during April.
The answer is a clear yes.
After an initial phase for healthcare and frontline workers, vaccination opened out to the elderly and over-45s with comorbidities on 1 March. The bulk of those vaccinated during March were the elderly, and by the end of March, around 44 percent of Mumbai’s elderly had had one vaccine dose. The great majority of these doses were of Covishield, the Oxford-AstraZeneca vaccine.
A 44 percent drop in fatalities in the elderly would correspond to a 34 percent drop across all groups.
Let’s summarise what we’ve found so far.
The rough match between these three, entirely different, calculations is remarkable.
Could it be that as hospitals filled up more and more COVID-19 deaths of the elderly at home went unrecorded? Have the elderly managed to ‘shield’ more successfully than younger people?
We can’t completely rule out such possibilities — but vaccine protection is the simplest explanation based on the scale and timing of the trends. Up to 580 lives could already have been saved by the vaccination drive focussed on the elderly.
What could we see next? From early April, vaccination opened up to all those over 45. Since then 45-60 year olds have received the bulk of new vaccine doses. With increasing protection in younger age groups, we could even see a slowing or reversal of the trend for an increasing fraction of fatalities in the under-60s.
This should not be alarming provided it comes alongside a reduction in COVID-19 deaths in the city as a whole. We can hope for some positive news in these grim times.
[A version of this piece with more data and details of the calculations is available here.]
(Murad Banaji is a mathematician with an interest in disease modelling. He tweets @muradbanaji. This is an opinion piece, and the views expressed are the author’s own. The Quint neither endorses nor is responsible for the same.)
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Published: 26 Apr 2021,06:15 PM IST