What is the data driving the UK’s new coronavirus restrictions?

Boris Johnson said infections were doubling between every 7 and 20 days - but scientists fear the rate is at the lower end of that 

Chris Baynes
Wednesday 23 September 2020 20:32 BST
Coronavirus deaths will rise to hundreds a day if second wave continues, Vallance warns

Boris Johnson’s announcement of stricter coronavirus rules came one day after senior government scientists Chris Whitty and Patrick Vallance warned the spread of the disease was “doubling roughly every seven days”

In a press conference on Monday, England’s chief medical officer and the UK government’s chief scientific adviser warned the country could see 50,000 new infections each day by mid-October and 200 deaths daily by mid-November if the spread of Covid-19 continued apace.

The figures were cited by Professor Whitty and Sir Patrick as they urged the UK to “change course” and paved the way for the prime minister to impose tougher restrictions such as the 10pm pub curfew and mandatory masks for shop staff.

However, in a speech to parliament on Tuesday, the prime minister said cases were doubling between every “seven and 20 days”.

That has led some to question whether the figures stated by Professor Whitty and Sir Patrick were an overestimate that exaggerated the need for urgent action. 

But Downing Street said the range cited by Mr Johnson reflected the moving picture presented by different sources of data being used to inform the government’s decisions.

The estimated doubling time of the virus is informed by work by the Scientific Advisory Group for Emergencies (Sage) modelling subgroup, which looks at the results of research such as the Imperial College London’s Real-time Assessment of Community Transmission (React-1) and the Office For National Statistics (ONS) infection survey.

Growth rates published by the government last week estimated the number of new infections was doubling between every 10 to 20 days. However, this reflected transmission up to three weeks ago and Sage advisers are concerned the doubling time may since have accelerated to only a week.

New ONS data published later this week will provide a clearer picture of the pace of the virus’s spread.  

But most of the recent available evidence points now towards the lower end of the range cited by the prime minister, Mr Johnson’s official spokesperson said. They added: “More data is being published still the time. Hospital rates have shown a doubling.”

Some experts have also suggested the rate of 50,000 infections a day cited by Prof Whitty and Sir Vallance is “implausible”, with University of Edinburgh epidemiologist Prof Mark Woolhouse noting it would “put the UK right at the top of the list of world’s countries affected by Covid-19”.

However, the government scientists emphasised in their press conference that the figures were “not a prediction” but worst-case scenario modelling which illustrated “how quickly this can move if the doubling time stays at seven days”.

Dr Flavio Toxvaerd, a lecturer on the economics of infectious diseases at University of Cambridge, said: “These projections often rely on behavioural assumptions that are unlikely to be borne out in practice and so the projections often look too gloomy in retrospect.

“The worst-case scenarios that are depicted in the graphs assume that people do nothing at all to protect themselves, something most epidemiologists agree is highly unlikely to be the case.  In practice, the expectation is that people will self-protect and thereby curb the epidemic somewhat.”

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