The R reproduction rate – a key measure of how much COVID-19 is spreading – has gone up in the UK in the past fortnight due to the epidemic in care homes, two leading experts have suggested.
It is a measure of how many people, on average, will be infected for every one person who has the disease.
If the R is one, then one person with the disease infects one other person.
As long as the R value is below one, the number of daily cases will continue to fall.
Sir Ian Diamond, the Office for National Statistics chief, said he agreed with earlier comments by scientist Professor John Edmunds who claimed it has risen from around 0.6 a few weeks ago because of the rapid spread in nursing homes.
"That is driven by the epidemic in care homes, he would say and I would not demur from that," Sir Ian told the Downing Street COVID-19 news conference.
Professor Edmunds earlier told MPs on the Science and Technology Committee that the R value is currently between 0.75 and one.
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And Foreign Secretary Dominic Raab, speaking at the government's daily briefing, said the R value was between 0.5 and 0.9.
"We have to lift measures gradually and monitor what happens," Prof Edmunds said. "It would be damaging to have to go back into lockdown.
"We can't rule it out. Measures might have to be imposed locally if an outbreak is spreading but I very much hope to avoid that."
Mr Raab said a "very significant issue" remains in care homes but the overall number of daily new infections and deaths were both "steadily falling".
However he cautioned: "The virus is not beaten yet, it remains deadly and infectious."
Prof Edmunds, who sits on the government's Scientific Advisory Group for Emergencies (SAGE), also warned cases need to fall substantially before contact tracing can be relied on to help prevent a second wave.
He said the system would be quickly overwhelmed if ministers relied on contact tracing once the lockdown is eased next week.
Social distancing will be essential to reduce the transmission of the virus, he added.
Prof Edmunds said: "We have perhaps 20,000 cases a day. To contact trace (the contacts of) all those would be an enormous undertaking. It's probably impossible at the moment.
"The incidence has to come right down for contact tracing to be feasible."
The government used contact tracing in an attempt to control the spread of the virus at the start of the UK outbreak.
But each case had been in close proximity with around 40 people, who needed to be urgently traced and isolated as a precaution.
The system was quickly overwhelmed, with large numbers of contacts who had been missed going on to develop COVID-19. The policy was abandoned on 12 March.
"If you were to get the incidence right down now then contact tracing would have a role," Prof Edmunds said.
"I would not want to rely on it alone so there will need to be other social distancing measures in place, so we don't go back to a situation where contact tracing is just scratching at the surface and the epidemic is out of control."
Also, new research shows that people are beginning to move around more as they anticipate a relaxation in the social distancing rules.
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