Britain will never truly know the reproduction number of the coronavirus and will have to squash it as they lift lockdown by trial and error, scientists say.
The reproduction number, also known as the R0 (R-nought), shows how many people the average patient infects before they recover.
Scientists say that as long as the rate is above one the outbreak will continue because the virus is still spreading faster than one-to-one in the community.
Chief medical officer, Professor Chris Whitty, said he believes it to currently be between 0.5 and one, meaning the epidemic is being forced to slow down.
As the Government tries to bring the country out of its current social distancing measures it must mix and match rule changes in a way that keeps the R0 as low as possible.
Prime Minister Boris Johnson, who today chaired his first Cabinet meeting since being hospitalised with the virus, is expected to confirm the slowed rate of infection and to vow that he will avoid doing anything that could take the infection rate to higher than one.
One expert told MailOnline that without a vaccine or herd immunity, controlling human behaviour will be the only way to stop the virus spreading out of control.
And widespread testing, contact tracing and tracking the number of people infected will be the only way officials can maintain a handle on how fast the illness seems to be spreading, although the meagre testing in the UK will only give a rough idea.
The number of people being diagnosed with the virus is falling over time despite increased testing, showing that the number of people each patient infects (the R0) has decreased since the lockdown began in March
Professor Chris Whitty, England’s chief medial officer, said at a science committee meeting with MPs last week that he thinks the R0 of the coronavirus is between 0.5 and 1
Dr Jennifer Cole, a biology expert at the University of Holloway in London, told MailOnline: ‘It’s [R0] not a set number. Some of it is down to the characteristics of the disease but about 80 per cent is due to the behaviour around it.
‘People’s behaviour has much more impact than biological factors so, rather than understanding the R0 and what factors affect it, it is important to understand the characteristics that play into the R0 and to calculate and plan how you bring that down.’
The R0, Dr Cole explained, can never truly be known because it’s fluid. It changes in different areas and different situations.
Mass, detailed contact tracing and mapping local outbreaks would be the best way to work out the virus’s R0, by showing who is spreading the illness and how.
And a wide net of testing which picks up people who have mild or no symptoms, as well as the sick and uninfected, would be the best way to track it over time.
Currently the UK has neither of the two, although it has in the past week announced plans for random population testing for thousands of people and an army of 18,000 people employed as contact tracers to follow the virus as it spreads.
These measures should give authorities enough of an idea about the rate of infection to be able to bring the nation out of its current lockdown, Dr Cole said.
This suggests it will not be possible to emerge from lockdown until these measures are put in place.
Mr Johnson is today expected to announce that the countrywide shutdown will continue into June at the earliest.
‘It’s incredibly difficult to calculate without doing it in retrospect,’ she said, explaining that detailed data can show how fast the virus has spread, but these do not relate to the present day.
‘At the moment we don’t have exact numbers but we have a rough idea and, as long as you can say the R0 is between one and two, or between three and four, that’s broadly enough to make the decisions you need to on social distancing.
The Imperial College COVID-19 Response Team, some of the Government’s foremost advisers on the crisis, put the average R0 at 2.4 in a paper published before lockdown.
This meant that, before Prime Minister Boris Johnson ordered the national shutdown on March 23, every 10 people who caught the virus would infect 24 others.
Mr Johnson is expected to announce tonight that this number is now below one, in his first Downing Street briefing since he was admitted to intensive care after catching COVID-19.
The PM is expected to extend lockdown until June and to insist he will put the R0 at the heart of the battle, refusing to do anything which pushes it above one.
But he may set out a blueprint for factories, construction sites and offices to return to work, including advising employers to avoid face-to-face meetings and to limit the numbers of people using canteens, the Financial Times reports.
Professor Whitty told MPs in a Science and Technology Committee meeting last week: ‘The R that we have at the moment is somewhere between 0.5 and 1.
‘Let’s say for the sake of argument it is in the middle of that range, which I think is likely, that does give a little bit of scope for manoeuvre and ticking some things off while still keeping it below 1.’
Researchers at the London School of Hygiene & Tropical Medicine said in a paper published at the start of April that they thought the number was 0.62.
They surveyed 1,300 people to ask about their movements and contacts and to judge how many people they were likely to have infected if they were carrying with the virus.
Government officials are soon expected to have to juggle lockdown-easing measures based on their likely impact on the coronavirus’s reproduction rate.
SAGE, the Scientific Advisory Group for Emergencies, led by Professor Whitty and Sir Patrick Vallance, the chief scientific adviser, will advise ministers on how different rule-changes could change the R0.
And Whitehall will be expected to cobble together a plan by adding together measures, each assigned an estimated value it would add to the R0, while trying to keep the total number below one.
Allowing people back onto buses and trains, for example, would be expected to push it higher than reopening parks, because people are forced into close quarters where they breathe on one another and share handrails, which can harbour the virus.
closed beaches while some didn’t, and [it turned out] beaches didn’t seem to have much of an impact on spread.
‘Subways do have quite a big influence on spread. In London, the tubes closed down quicker than the buses did and that seems to have been the right decision.
‘Instead of reopening the Underground it might be better to put on more buses.’
Germany is now facing the prospect of having to re-tighten its lockdown restrictions after loosening them too soon appeared to allow the R0 to rise too far.
The number of new COVID-19 cases there yesterday rose the most it had for four days and Germans have been urged to stay at home to stop a second wave.
Its official disease control department, the Robert Koch Institute, yesterday estimated that the coronavirus’s R0 in Germany is 0.75. It had been 0.9 on Tuesday.
The RKI estimates its R0 using testing figures and comparing rolling differences in how many people are testing positive for the virus.
It is able to do this because it has a superior testing regime to much of Europe, carrying out an average of 400,000 tests per week for the past month.
Britain, by comparison, carried out 170,000 tests last week on 120,000 people and has done a total of 818,539.
The Government is increasing its capacity for testing – although it is not expected to hit its target of 100,000 tests per day by tomorrow – and introducing random population sampling in a bid to get a better picture of the size of the outbreak.
Professor Whitty said he believes the country now has antibody tests – which show who has already recovered from the disease – that are good enough to track how many people have had COVID-19 already.
Speaking on lifting the lockdown in last week’s science committee, he said: ‘We are [not] suddenly going to be able to lift everything, but nor do I think it likely that we will have to keep in exactly the current pattern for the indefinite future.
‘There’s somewhere between those two, and working out exactly what that is, and what the timescale of that is, and what the package is, is going to be a difficult task for governments all around the world, and certainly obviously including the UK.’
Tracking how many people are infected and controlling the movement of people will be the only ways to keep the virus under wraps until a cure or vaccine is found.
Dr Cole said: ‘Bringing the R0 below one is what you need to do for the outbreak to die out. It just dies out because there aren’t enough people to pass it on to.
‘That also comes with herd immunity and vaccination, but without any medical intervention the way that you bring R0 down is for people to stay away from each other.’
She explained that chickenpox is kept under control by herd immunity – so many people develop immunity as children than it cannot spread among adults – and measles by vaccination.
But neither of these exist yet for coronavirus.
It would take around 62 per cent of the population to be infected for herd immunity to develop for COVID-19, according to the University of Oxford’s Professor Carl Heneghan, but there is no guarantee people cannot catch it twice.
Vaccines are in development – notably one made by the University of Oxford is in human trials already and an Imperial College trial starts in June.
‘It’s the perfect storm – quite infectious and quite deadly,’ Dr Cole added. ‘Most deadly viruses are very hard to catch and equally most viruses are very mild.
‘If you’re a mild irritation people aren’t going out of their way to exterminate you; if you’re very annoying and very deadly they will. COVID-19 at the moment kind of sits in the middle of that.
‘If it wants to survive and not be eradicated it needs to make a deal with nature – they do this by evolving to become less deadly.
‘Swine flu did this and nobody really notices it any more. That may or may not come [for COVID-19] before the vaccines are available.’