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The UK is introducing stronger measures to tackle the spread of the coronavirus, but some of the science that helped to inform this approach suggests it still won’t be enough to avoid a large number of deaths.

Yesterday, UK prime minister Boris Johnson said people should stop all unnecessary travel, and work from home if possible. He said pubs, clubs and other social spaces should be avoided and whole households should self-isolate for 14 days if any individual in it develops covid-19 symptoms. People who may be particularly vulnerable to the virus may soon be asked to stay home for 12 weeks.

These new policies were informed at least in part by that modelled the potential of various public health measures to keep a lid on the spread of the virus until a vaccine is available, which may still be 18 months or more away. “Our main focus is to understand, at this current time, what are the potential impacts of various measures,” says , who discussed the report at a press conference on Monday.

There are two basic strategies available for tackling the coronavirus: mitigation, which aims to slow but not necessarily stop the epidemic, and suppression, which aims to reduce transmission to a minimum and keep it that way so that the epidemic dies out. Suppression is the best option for public health but is extremely difficult to achieve and maintain. “This has never been tried anywhere ever,” says Tim Colbourn at University College London. “Can it work?”

An “unacceptable” level

Ghani and her colleagues calculated that mitigation, which reduces the demand on health services and can help protect vulnerable people, would still overwhelm the health system and lead to an estimated 250,000 deaths in the UK, about half of the number that would occur with no control measures. She describes this as “probably still an unacceptable level of burden”.

Nonetheless, the UK government still seems to be leaning towards mitigation, rather than suppression, for now. The modelling shows that suppression would require a bare minimum of three measures: home isolation of all cases for seven days, quarantining of all their household members for 14 days, and population-wide social distancing. This is defined as a 75 per cent reduction in social contact outside home, school or work. “The sorts of contacts one makes in pubs, theatres, restaurants,” says Ghani.

While the new measures in the UK go some way towards this, some say they leave too much wiggle room. “It is clear that social distancing needs to be happening now,” says Alan McNally at the University of Birmingham, UK. The new measures “begin that process”, he says.

The modelling also suggests that school and university closures may be necessary to achieve suppression. This package of measures would have to be maintained for many months, probably until a vaccine is available. It could be temporarily relaxed from time to time but would have to be reimposed whenever infections start rising again.

Daunting challenges

The lesser option, mitigation, also requires home isolation and quarantine, but social distancing only for the over 70s and no school and university closures. The models suggest that this will result in an epidemic that will peak over the spring and summer, lasting between three and four months. This “flattening of the curve” will reduce peak demand on health services by two-thirds, says Neil Ferguson at Imperial College London, who led the analysis.

But even this level of self-discipline will be hard to maintain. “The challenges we collectively face are daunting,” says team member London. “However, our work indicates if a combination of measures are implemented, then transmission can be substantially reduced.”

However, “there are still huge uncertainties around any future estimates”, says Michael Head at the University of Southampton.

The team also modelled the impact of mitigation in the US and concluded that it would roughly halve the number of deaths there, from to 2.2 to 1.2 million.