The science behind the big shift in the UK’s coronavirus strategy

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Late on Monday afternoon, Boris Johnson announced a series of far-reaching recommendations that signaled a dramatic ratcheting up of the UK’s coronavirus response. All UK citizens are now advised to avoid pubs, restaurants, theatres and bars while those who are over 70 or who have underlying health conditions are advised to significantly limit all face-to-face interaction where possible.

While this stops short of the total lockdowns in place in Italy, Spain and France, it’s a big difference from a week ago, when the main government advice focused on handwashing and those with symptoms isolating for seven days. So what changed?

One influence was a report from Imperial College London which set out the potential impacts of two different strategies for combating the coronavirus, Covid-19. The first strategy – which the report calls mitigation – involves isolating suspected cases, quarantining households with suspected cases and socially distancing those most vulnerable for around three months at the peak of the outbreak. Such an approach would still result in “hundreds of thousands of deaths” and leave intensive care units overwhelmed “many times over,” the authors of the report wrote.

Even in a best-case scenario, following a mitigation approach would exceed critical care bed capacity by more than eight times their surge capacity. The models used by the Imperial epidemiologists estimated that this would lead to 250,000 deaths in the whole of Great Britain. “In the UK this conclusion has only been reached in the last few days, with the refinement of estimates of likely ICU [Intensive Care Unit] demand due to Covid-19 based on experience in Italy and the UK,” the authors wrote.

That leaves the UK with only one viable strategy, the authors conclude: suppression. While mitigation is an attempt to slow, but not entirely stop, the spread of the outbreak, suppression aims to reverse the epidemic growth altogether by reducing case numbers and keeping them down. The mitigation policies the Imperial scientists modelled were in place for three months, but suppression policies would need to stay in place for at least five months.

Suppression is a significant step-up from mitigation. It includes the social distancing of the entire population with the added possibility of school and university closures. “A minimum policy for effective suppression is therefore population-wide social distancing combined with home isolation of cases and school and university closures,” the authors write. To avoid a spike in cases when suppression is eased, the policies will also need to be in place until enough vaccines are available to immunise the population, which could be at least 18 months.

The most effective suppression strategy according to the report is a combination of social distancing of the entire population, isolation for people with symptoms, household quarantine and school and university closures. While schools remain open, many universities have taken the decision to move teaching online. But successful suppression also has an unfortunate side effect: the number of cases will likely surge once suppression measures are lifted because most of the population will still not be immune to the disease. One way to mitigate this, the authors suggest, is to trigger suppressions intermittently – turning them on when intensive care beds exceed surge capacity and relaxing them once the pressure eases again. This would reduce the demand on ICU beds, but it would mean suppression policies being in place for the majority of an extend period of time.

At the heart of both of these strategies is one number: 5,000. That’s the total of available ICU beds in Great Britain in surge capacity mode. Excess deaths start to ramp up when these beds hit capacity as those who need critical care are not able to access it – which is exactly what has happened in Italy. The Imperial report estimates that around 4.4 per cent of people infected with Covid-19 will need to be hospitalised, with 40 per cent of those people requiring critical care. For the most vulnerable people that figure is much higher. Around 27.3 per cent aged 80-plus who show symptoms will require hospitalisation, with 70.9 per cent of that number require critical care.

The Imperial report sets out what will need to be done to suppress the Covid-19 outbreak, but what remains to be seen is how long dramatic social distancing measures can be kept up. While China and South Korea have shown that it’s possible to get on top of Covid-19 in the short term, we still don’t know what will happen when those measures are relaxed. What we see there could well determine the shape of the UK’s future response.

Matt Reynolds is WIRED’s science editor. He tweets from @mattsreynolds1

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