THE R rate is one of the key metrics used to guide public policy on the coronavirus pandemic – including whether to go into lockdown.
But the data used by Government scientists to determine the estimated weekly figure has a time delay of up to three weeks, experts have warned.
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It means that it could be the end of the month before a national lockdown starts to show any impact on the crucial figure.
The R value is the number of people an infected person will transmit the disease to, and is based on estimates, shown as a range.
On Friday, the Scientific Advisory Group for Emergencies (Sage) estimated the UK’s R rate to be between 1.1 and 1.3.
Separate research by Imperial College London, published on Thursday, suggested the value could be much higher – at almost 3 in London and above 2 in the South.
On Saturday, Boris Johnson the country will go back under strict measures for four weeks after local restrictions failed to sufficiently reduce infections.
However, there was some suggestion it could be extended beyond December 2 after Cabinet Office Minister Michael Gove admitted they may need to be in place for longer.
Experts warn it will take time before any change in restrictions is reflected in the number of infections and ultimately the R rate.
Professor Paul Hunter, of the University of East Anglia, said: “Although we should see evidence of decline in cases after a couple of weeks, I doubt that case numbers will have fallen dramatically by the end of the four week period and if all restrictions were then relaxed it would not be long before case numbers started to increase again.”
Sage says it uses several models, each using data from a variety of sources, but warned that means there is a time delay on their estimates.
“Epidemiological data, such as hospital admissions, ICU admissions and deaths, usually takes up to three weeks to reflect changes in the spread of disease,” they said.
“This is due to the time delay between initial infection, having symptoms and the need for hospital care.”
Sage also uses household infection surveys – where swabs are performed on individuals – which can provide estimates of how many people are infected.
However, surveys – such as the one carried out by the Office for National Statistics (ONS) – can take up to a week to be collated and provide results.
Another data source includes contact pattern surveys that gather information on behaviour, but these also have a lag of around a week.
Professor Kevin McConway, a statistician at The Open University, said: “A limitation of the Sage results is that they are based on a range of data including data on hospital admissions and deaths.
“Since it takes some time after a person is infected for them to require hospital admission or, sadly, to die, this means that effectively the estimates of R and the growth rate are based on average changes over, roughly, the past three weeks.
“This would only be important if the pattern of growth changed a lot over that period.”
A Sage spokeperson said: “The latest published figures represent the situation over the past few weeks rather than today.
“These estimates do not yet fully reflect any very recent changes in transmission due to, for example, recent policy changes in the UK.”
‘2-3 WEEK DELAY’
Professor Neil Ferguson, whose modelling prompted the first lockdown in March, said it will take “probably two to three weeks” to see whether another shutdown will lead to a reduction in hospital admissions.
“The actual reduction in the R should happen basically as of Friday,” he told Times Radio Breakfast.
“(If) People reduce their contacts as we expect them to, then that will be immediate, but it takes time for it to filter through into reduced admissions to hospitals, reduced deaths per day.
“That will take time, so it will take us probably two to three weeks to see an effect on numbers.”
Asked if it would be helpful for schools to close, Prof Ferguson, of Imperial College London, said: “From a purely epidemiological perspective, getting the transmission down, clearly it would, it would reduce transmission.
“But you have to counter-balance that with the harms closing schools do, and that fundamentally is why we have politicians making those judgements.”
Prof Ferguson also said that driving infection levels down “substantially” could see measures relaxed over Christmas, but acknowledged this “poses some risks”.
“If we can drive infection levels down substantially, then we’ll be in a better position to relax things over Christmas than if they are still at the current levels,” he told Times Radio Breakfast.
“Relaxation will inevitably pose some risks, it will lead to more transmission, but, if that’s from a low baseline level, the costs of that and the harms it causes are going to be less than if we were at current levels.”
He said “on-off measures” were always anticipated to be an outcome from the pandemic, adding: “Clearly, in some ways it will be better to find the perfect set of measures to keep transmission under control without needing to go in and out of lockdown.
“It is quite possible, unfortunately, that we may have to revisit this again come early January or something.
Any change in restrictions takes about two weeks before any impact is seen on numbers of new cases
Prof Paul Hunter
“I very much hope we don’t, but I think everybody we have spoken to says we have to see what the data shows in the next few weeks, and adjust the policy accordingly.”
Professor Paul Hunter, from the University of East Anglia, said: “These new restrictions have been implemented for four weeks.
“The big question is whether they will be sufficient to reverse the rapid increase we have seen in recent weeks.
“Any change in restrictions takes about two weeks before any impact is seen on numbers of new cases.
“As we saw in April and May, even with the full lockdown we had then, case numbers declined much more slowly than they had increased.
“Given that schools are remaining open it is likely that any decline will be even more gradual now than in April.
“Although we should see evidence of decline in cases after a couple of weeks, I doubt that case numbers will have fallen dramatically by the end of the four week period and if all restrictions were then relaxed it would not be long before case numbers started to increase again.”