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The UK only realised "in the last few days" that a strategy to slow down the spread of coronavirus could lead to 250,000 deaths.

The verdict has emerged in a new academic study released last night, hours after Boris Johnson drastically ramped up his approach to the COVID-19 outbreak.

Its authors looked at Italy and decided their "worst case scenario" was actually "the most likely scenario".

Britain's government had until yesterday focused on "delaying" and "mitigating" the spread of the virus, which has a death rate of around 1%.

But last night the Prime Minister announced the biggest restrictions in peacetime - ordering people to avoid pubs, restaurants and clubs, unnecessary travel, and leaving the house where at all possible.

The ramping up followed a study by the Imperial College Covid-19 response team, which has been advising ministers.

The experts modelled two scenarios - merely "mitigating" the virus, and trying to "suppress" it completely like in China.

The study found suppression is "the only viable strategy at the current time", because the alternative would exceed the demand for hospital and intensive care beds eight times over - even in "the more optimistic scenario".

(Image: Getty Images)

Under mitigation only, it added: "Even if all patients were able to be treated, we predict there would still be in the order of 250,000 deaths in GB, and 1.1-1.2 million in the US."

The study went on: "In the UK, this conclusion has only been reached in the last few days, with the refinement of estimates of likely ICU demand due to COVID-19 based on experience in Italy and the UK.

"Previous planning estimates assumed half the demand now estimated.

"And with the NHS providing increasing certainty around the limits of hospital surge capacity."

Despite recommending suppression "imminently" in the UK, the study admitted it will have "profound" social and economic effects - and no end date.

The draconian measures "will need to be maintained" until a vaccine becomes available, the study says, a moment could easily be 18 months away.

Otherwise the infection will swiftly rebound, the study warns. There could be "temporary" relaxations of restrictions in "short time windows", but no more.

Deputy Chief Medical Officer Jonathan Van-Tam told BBC Radio 4’s Today programme: “We can’t say how long this will need to go on for.

“I don’t know if it could be a year yet. I think we’re too far out to make those kinds of predictions. But I certainly think it could be several months.”

The later it is introduced the harder the virus is to suppress - as there is a lag time of "two to three weeks" before cases present in hospital. The UK already has 1,543 confirmed cases despite not testing all those who show symptoms in the community.

"There are very large uncertainties around the transmission of this virus, the likely effectiveness of different policies and the extent to which the population spontaneously adopts risk reducing behaviours," the study says.

"This means it is difficult to be definitive about the likely initial duration of measures which will be required, except that it will be several months. Future decisions on when and for how long to relax policies will need to be informed by ongoing surveillance."

The study admits suppression "may need" to be supplemented by shutting schools and universities, but "closures may have negative impacts on health systems" because more parents are forced to stay away from work.

If the epidemic was completely unmitigated, not something the government has ever considered, the death toll would be estimated at 510,000 and the need for intensive care beds would be exceeded 30 times over.

The study also contained new data on death rates suggesting a huge gulf between the old and the young.

The fatality rate is estimated at just 0.002% for under-10s and 0.08% for those aged 30 to 40.

But it rises to 2.2% for those in their 60s, 5.1% for those in their 70s, and 9.3% for those over 80.

Prof Neil Ferguson of Imperial College admitted UK chiefs changed tack when they realised the “worst case scenario” was actually “the most likely scenario”.

The expert - who led the study warning of 250,000 deaths - told BBC Radio 4’s Today programme: “It’s not so much changing our conclusions, we don’t recommend policy - it has been a refinement of estimates.

“Initially when we came up with these sorts of of estimates, they were viewed as what’s called the reasonable worst case scenario.

“But as information has been gathered in recent weeks, from particularly Italy and other countries, it’s become increasingly clear that actually this is not the reasonable worst case. this is the most likely scenario.”

The other “critical” piece of information was when NHS planners came up with how much they could expand surge capacity.

“Whilst they are planning a major expansion of that… it just isn’t enough to fill the gap which would be left. So we are left with no option but to adopt this more draconian strategy,” he said.

As per the Imperial report, he warned it would take two to three weeks before the UK’s measures start being reflected in case numbers.

He defended the timing of the move but said: “There wasn’t any time to lose."

The stark warning came after Boris Johnson on Monday unveiled unprecedented peacetime measures to try to control the spread of Covid-19.

They were announced as the death toll of people with coronavirus in the UK reached 55.

In the first of his daily No 10 press conferences, the Prime Minister called on people to stay away from pubs, clubs and theatres and to avoid all non-essential contacts and travel.

In other developments:

Chancellor Rishi Sunak will set out a new package of support for businesses hit by the outbreak less than a week after announcing £12 billion of emergency funding in the Budget

The Government will lay out emergency legislation expected to include powers for police to detain people to stop them spreading the virus and allowing hospitals to send patients home to free up beds

All non-essential access to the Houses of Parliament has been stopped and MPs and peers over the age of 70 or with underlying health problems have been urged to heed Government advice

The Prime Minister has held a conference call with manufacturing firms and organisations urging them to step up production of ventilators and other vital medical equipment

Under the latest Government advice, anyone living in a household with somebody who has the symptoms of a persistent cough or fever was told to isolate themselves for 14 days.

Special guidance will be issued by the NHS for the 1.4 million people most at risk from the disease - including the elderly with underlying health conditions - on further measures they need to take to "shield" themselves.

Mr Johnson said the measures were needed as the UK was approaching the "fast growth part of the upward curve" in the number of cases.

"Without drastic action, cases could double every five or six days," he said.

However, the Imperial College report warned that even with such a dramatic closing down of normal life, the capacity of health systems in the UK and the US - which is adopting similar measures - was likely to be "exceeded many times over".

Even then, it said it was "not at all certain" that the strategy would succeed in the long term.

"The social and economic effects of the measures which are needed to achieve this policy goal will be profound," it said.

"No public health intervention with such disruptive effects on society has been previously attempted for such a long duration of time. How populations and societies will respond remains unclear."

(Image: Richard Pohle)

In response, a Government spokesman said the recommendations put forward by its Scientific Advisory Group for Emergencies (Sage) were in line with "best current evidence".

"This is a very fast-moving situation. In order to give the most robust scientific advice Sage draws upon and considers a range of evidence and views to reach its recommendations," the spokesman said.

"Part of this evidence includes the latest modelling data from a number of experts. All Sage recommendations are in line with the best current evidence. We will be publishing further evidence shortly."

At his news conference Mr Johnson the said the latest Government measures represented a "very substantial change" in the way it was asking people to live their lives that was unprecedented in peacetime.

He said they represented "a very considerable psychological, behavioural change" but he said he had "absolutely no doubt" the country could do it.

The Government's chief scientific adviser Sir Patrick Vallance insisted the measures would have a "big effect" on the spread of the virus.

"This is not a series of small interventions. You would anticipate that this could have a dramatic effect to reduce the peak and to reduce death rates," he said.