Coronavirus | Countries report less than 20% of symptomatic cases, says U.K. epidemiologist

U.K. epidemiologist Adam Kucharski says a big challenge in fighting COVID-19 is that a lot of transmission happens very early on in the infection

March 28, 2020 10:57 pm | Updated December 03, 2021 06:39 am IST

Adam Kucharski.

Adam Kucharski.

Professor Adam Kucharski, who is an Associate Professor and the Sir Henry Dale Fellow at the London School of Hygiene and Tropical Medicine, specialises in the mathematical analysis of infectious disease outbreaks. He spoke to The Hindu about the global coronavirus pandemic, specifically on what nations can learn from models of infectious outbreaks, and even of fake news. Edited excerpts:

In your book, The Rules of Contagion: Why Things Spread – and Why They Stop , you talk about Ronald Ross, a Nobel Prize-winning British doctor who in the late 19th century discovered that mosquitoes spread malaria. How can we apply the blend of mathematics, biology and societal analysis that he did to predict the endgame scenario for the coronavirus pandemic today?

One of the key insights that Ross made was regarding the control of infection. People had this idea that you couldn’t control malaria until you could remove every single last mosquito. Ross used these simple, conceptual models and realised that actually you don’t need to remove every last mosquito. Once you get the density of mosquitos low enough, the chances are that someone who is infected would recover before they pass it on to others.

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We can think of a similar idea with a lot of the physical distancing measures that are coming in. Initially, there may still be some infectious people out there, but because these measures are in place, they will have the infection and recover before they pass it on to someone else. That’s what we’ve seen in places like Wuhan.

How, in terms of the mathematics of epidemiology, does COVID-19 differ from other major infectious outbreaks such as the Spanish flu, SARS or Ebola?

There are a number of broad principles that we can use to understand these infections. These can apply across a number of pathogens, particularly in understanding the magnitude of spread. So, each person who gets infected, on average, how many people are they giving the virus to? For COVID-19, it is about two or three. We can also think about the time scales: if you have one case, how long on average is it until the person infected then shows symptoms? For COVID-19, that’s about five days or so.

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One of the big challenges or differences of COVID-19, as opposed to SARS or Ebola, is that a lot of transmission seems to be happening very early on in the infection, when people don’t have symptoms or have very mild symptoms. One of the reasons why infections like Ebola and SARS have been easier to control is that a lot of people who are highly infectious have very distinctive symptoms.

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That means you can identify them, look at whom they’ve come in contact with, and make sure those people have been quarantined. However, for COVID-19, a lot of the transmission happens among people who might feel perfectly well, or might have a slight cough, for example. That makes it very hard to pick up all the infections, and is why we’ve seen in many countries, even if they’ve been trying to detect cases coming in, many cases have managed to go undetected.

India has a relatively low number of confirmed cases and correspondingly fewer recorded deaths linked to COVID-19. Given the size and density of our population, does this seem a little suspicious to you?

I think in the early stages, it is quite hard to untangle those two things, when you have a very small number of cases — is it just by chance, or population structure or other features that mean that transmission hasn’t quite taken off yet?

Once we start to get more severe cases, especially deaths, you can then start to get a clearer picture of how much infection you might be seeing. We recently did some rough estimates and we think that many countries that have clear outbreaks now are probably reporting less than 20% of their symptomatic cases.

Do you see any parallels between deadly pandemics such as COVID-19 and how fake news transmission happens?

There certainly are parallels. For COVID-19, on average, in the early stage, each case would give the infection to a couple of others. In a study of viral Facebook content, it was found that each person on average who shares a viral post will lead to about two more people sharing it. But the big difference is the time scale.

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With COVID-19, it takes a few days for that transmission and that new infection to occur whereas online we might be talking about 30 seconds.

Also, for COVID-19, there have been some outbreaks where a single event sparks a large number of secondary infections. A lot of content that becomes popular online tends to have that feature too. We see messages spreading on WhatsApp, but often there will be some high-profile person online, or some media outlet that will have sparked the outbreak and shared with a large number of people. Then you see these smaller but secondary clusters of sharing happening.

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