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What we scientists have discovered about how each age group spreads Covid-19 | Opinion


Early detection and early response. In an ideal world, this will contain an outbreak of infectious disease and prevent it from establishing in a population and becoming an epidemic.

Containment typically involves isolating and testing suspected cases, tracing their contacts, and quarantine in case any of these people are infected. The more thorough the testing and contact tracing early on, the better the chance of containment. These measures work well, especially for diseases like Sars, where symptoms coincide with infectiousness – it is easy to recognise people who are infected.

In the case of Covid-19, detection is much harder because some transmission is possible before people show obvious symptoms, in this case a fever and cough. If there are no symptoms, it is impossible to identify people who are infected unless they are tested and their infection is confirmed in a laboratory, which is why airport screening at arrival is not very effective for Covid-19. This makes the virus much harder to control. Sooner or later, an infected case escapes detection and starts a chain of transmission, and quickly leads to a growing number of cases, which can become impossible to contain. As a result, containment has failed in a number of countries and the World Health Organization has declared a pandemic that is here to stay for months, if not longer.

So how does an epidemic spread through a population – and how do we control it? In outbreak analysis, we measure transmission using the reproduction number (otherwise known as R), which tells us how many other people a typical Covid-19 case will infect on average. If one person is expected to infect more than one other person, the infection will grow and create an outbreak; if a person infects less than one other, the infection will peter out.

The larger the value of R, the easier the virus spreads through a population and the higher the number of infections. For seasonal flu, one infection leads on average to 1.4 new infections. For Covid-19, one infection leads to 2-3 others in the early stages of an outbreak. How many other people we end up infecting depends on several things: how many susceptible people there are in the population, how long we are infected for (the longer we are infected the more chances there are to pass on the infection), the number of people we come into contact with, and the probability of passing on the infection to those contacts.

In order to control the epidemic, we need to reduce R below 1. For diseases such as measles, we can achieve this by vaccinating a large enough proportion of the population so that the disease can no longer spread. Exactly what proportion we need to vaccinate to reach community immunity depends again on R. For R of 2, we need to immunise half of the population to interrupt transmission; for R of 3 we need to immunise two thirds of the population to get R below 1.





Covid-19 test kit



‘In order for tests to be most effective, results need to be processed quickly (in the UK it takes 2-4 days to get the results, in Wuhan it is under 4 hours).’ Photograph: Christopher Furlong/Getty Images

However, an effective vaccine for Covid-19 is unlikely to be available for another 12-18 months at best.

Shortening the duration of infection could also reduce transmission, and for some infections, such as influenza, this can be achieved with antiviral therapy. There are currently more than a hundred clinical studies in progress, which could produce therapies that reduce the duration of infection or infectivity. However, both of these control options are not yet available.

Another option to reduce the duration of infection is continued testing; this is a measure that can be effective throughout the epidemic. Finding infected people and advising them to self-isolate for the duration of the infection minimises the risk of them passing on the infection, which therefore slows down the spread. This is again a basic intervention method that we know works well, but in order for it to be most effective, results need to be processed quickly (in the UK it takes 2-4 days to get the results, in Wuhan it is under 4 hours).

If we can’t easily reduce susceptibility or duration, we are left with two things: the probability of transmitting the infection upon contact and the number of different people we come into contact with. Both of these are things we as individuals can change by modifying our behaviour. Washing hands frequently for 20 seconds with hot water and soap, not touching our faces, covering our coughs and sneezes, staying at home if we are sick – all of these reduce the risk of getting infected or passing on the infection to someone else.





A nearly empty tube carriage during rush hour in London.



‘If those of us who can work remotely start doing so now, it will contribute to lowering overall transmission in the population.’ A nearly empty tube carriage during rush hour in London. Photograph: Justin Tallis/AFP via Getty Images

Cancelling large gatherings, working from home and school closures are all social distancing measures aimed at reducing the number of contacts between people, each of which is an opportunity for passing on an infection. But this will not be a simple case of just cancelling events and closing schools for a couple of weeks. More severe measures will be necessary to bring the infections to low enough levels to keep the pressure off the health-system long-term. As individuals, we must all reduce the number of different people we come into contact with on a daily basis, and be prepared to do so for a prolonged period of time.

But what sorts of contacts are most important for transmission? Together with Adam Kucharski, also from the London School of Hygiene and Tropical Medicine, I have recently collaborated with the BBC on a massive citizen science project, led by Professor Julia Gog from Cambridge University. Called BBC Pandemic, the project collected information on how people of different ages interact with one another in different contexts (home, work, school, other) from over 35,000 volunteers. We have recently been fast-tracking the release of contact data to help inform UK Covid-19 modelling efforts and potential intervention strategies.

What we have found in this data is that adults aged 20-50 make most of their contacts in workplaces. If those of us who can work remotely start doing so now, it will contribute to lowering overall transmission in the population. Another important finding is that people over 65 – who are particularly at risk from severe Covid-19 illness – make over half of their contacts in other settings (not home, school or work), such as shops, restaurants and leisure centres. By avoiding these interactions, people who are most at risk from the new coronavirus could halve their risk of infection. By changing our behaviour now, and sustaining these changes throughout the outbreak, we can significantly reduce our own risk of infection, and the risk to others, and by doing so help protect those most vulnerable.

Sustainability is key here; these measures may reduce the reproduction number R, but as soon as they are lifted we could see transmission again, and another outbreak.

We must all take individual action to reduce Covid-19 transmission and slow down its spread, limit the number of infections and reduce the pressure on the health system so that everyone who needs medical care can get it. Starting now, and for the duration of this epidemic, we all have a role to play and a responsibility to modify our behaviour in order to protect the ones who are most at risk.

Dr Petra Klepac is assistant professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine

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