For how long can we maintain social distancing?

Modelling the coronavirus outbreak suggests that suppression is the only way to reduce fatal cases and allow hospitals to continue to function

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Nullius in verba is the motto of the Royal Society, the academy of scientists in the UK and publishers of the world's oldest journal. Roughly translating to "take nobody's word for it", it is an expression of enlightenment values. Don't believe me because I am rich, famous, or clever, believe me because I will show you the evidence.

It is therefore perhaps unsurprising that many scientists and non scientists alike loudly called for the UK government to publish the evidence for its coronavirus strategy. Why were public events still going ahead? Why were schools still open? When the advice changed earlier this week (although it feels like about a month ago) the advice was finally published in the form of a paper on the website of Imperial College London.

The MRC Centre for Global Infectious Disease Analysis, led by Neil Ferguson, are the politicians' favourite epidemiologists. They influenced the response to the Foot and Mouth Disease Virus outbreak during the Blair years, and their advice led to the famous postponing of the general election in 2001. They have developed a mathematical model of the UK and US populations, including individual social interactions, which allows them to show how a disease might spread. They can then tweak variables, such as closing or opening schools, limiting social contacts, closing workplaces, etc.

Their paper, which has not yet (to my knowledge) been peer reviewed, presented three main options: no measures taken to stop the outbreak; a mitigation strategy to slow the outbreak, but not stop it, so that the population slowly catches the infection and builds up immunity; and a suppression strategy to try to stop the outbreak, at least temporarily. It's worth going through each scenario in turn.

If no measures are taken, then the model predicts the outbreak to peak in around 3 months time and cause 500,000 deaths in the UK and 2.2 million in the USA. Those infected will have some immunity to the virus and so the number of infections will fall, as the virus struggles to spread further into fresh hosts. Outbreak over, but at a very high cost.

The mitigation strategy, formerly adopted by the UK government, involves isolating those who have symptoms such as a cough or fever, and reducing, but not stopping, social contact. This halves the number of predicted deaths in the UK, which is good news, but leaves them at an unacceptably high number of 250,000. This is at least ten times more than would usually die from seasonal influenza in the UK, which kills anything between 2000 - 30,000 mainly older people. Crucially, the pressure on hospitals would also be very high, with the intensive care unit (ICU) capacity in the NHS of 200 per week being regularly exceeded. The lack of hospital beds and ventilators had an understandably large influence on the government decision. For those who are hospitalised, they may need artificial ventilation. If there aren't enough ventilators or beds in ICU, then those who could have survived severe disease, may not.

The mitigation strategy has therefore been abandoned, and the new approach is a suppression strategy, which is the most draconian. This aims to push the reproductive number (R0) of the virus below 1, meaning that each infected person will infect less than one other person. The R0 of SARS-CoV-2 is around 2.2 (based on the Diamond Princess outbreak), so pushing this below 1 is quite a challenge. Other viruses can be more or less infectious: the R0 of measles virus is a stonking 15; the R0 for seasonal influenza is around 1.3. If you tweak the model so that there is social distancing of the whole population, then the R0 is predicted to dip below 1 and the epidemic is brought to a stop, with the NHS able to cope with the smaller number of people hospitalised each week.

However, the authors caution that the more effective the suppression, the bigger the outbreak when social distancing stops. The outbreak is effectively on pause, and as soon as you click play, by people returning to work, there is an outbreak just as big as there would be with no policy at all. This is because there is no herd immunity to the virus, as very few have been infected, and so few are protected from re-infection.

Suppression must continue until the virus can be tackled through one of a few options. The first is anti-viral drugs: HIV drugs were used in China, but with no effect; an experimental lung drug is being tested in Southampton, UK; and the WHO has identified several other candidates. The second is a vaccine: several have been developed and human trials have started in Seattle, but a safe vaccine is likely to be 12-18 months away. Vaccines, unlike anti-virals, are given to healthy people, so its safety is critically important. The third option (and they are not mutually exclusive) is that each country increases its capacity to deal with the severely ill: more hospital beds and ventilators. 

Can we operate such social distancing until we have anti-virals, vaccines, or more hospital capacity? The Imperial College model shows that social distancing needs to be in place for two thirds of the time, until a vaccine is developed. So this may mean two months of social distancing, say April and May, and then June starting to return to normal. But then August and September would also require social distancing again, so that the virus doesn't return.

All eyes are now on China, which has cleared the virus from its population through an unprecedented policy of suppression. As of today, there were only 34 new cases of coronavirus SARS-CoV-2 in the country, all of which were imported. China can now downgrade to a mitigation strategy, with testing and isolation of suspected cases and their contacts, and life starting to return to normal for the rest of the population. The world watches to see if the virus returns.

If you are now working from home, as many are, start planning for a long stay. If you don't have any symptoms, leave the house for a walk or a jog, perhaps take up slow cooking or gardening. I planted a winter jasmine on my balcony yesterday, delivered by post, and am already thinking of which rooms to paint. Social distancing is the new normal, for this year, and perhaps longer.

Image credit: Image by RD from Pixabay

Ben Johnson

Magazine Editor, Nature Medicine, Springer Nature

I trained as a virologist, starting with an undergraduate degree in virology from the University of Warwick, UK. My PhD, in influenza virus genetics and immunoevasion, was from Public Health England and the University of Reading, UK, with Maria Zambon and Wendy Barclay. My research interests then moved to smallpox vaccines, viral ion channels and cell adhesion, while a postdoc at Imperial College London with Geoffrey Smith, FRS. I then joined open-access publisher BioMed Central in 2011 as an editor and then associate publisher and was Head of Communities & Engagement at Springer Nature from 2016, running the Nature Research Communities and other online engagement activities for researchers. I joined Nature Medicine in 2021, with responsibility for news and opinion content, and am based in the London office.