Professor Neil Ferguson said today that Public Health England were unable to prevent two thirds of people from spreading the disease.
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The number of deaths rose by 700 yesterday – the highest amount since the crisis began – but those are likely to have been people who were infected weeks ago.
Britain’s total lockdown only started on March 23.
Flights were never stopped from China and Italy during the early stages of the pandemic, meaning people were still able to come in with no checks.
People kept carrying on coming into the country and were only told they must isolate if they develop symptoms.
It compares to other countries who forced people to isolate when they entered the country.
Professor Ferguson told the BBC’s Andrew Marr Show this morning: “People flew into this country and seeded it into this country…
“Public Health England did their best to isolate but only about one third of infected people were stopped.”
He said those people spread the disease across the country, and it was likely helped by countries like Italy who saw community transmission of the virus very early on.
Professor Ferguson – who has been helping advise the Government on what measures are needed to tackle the virus – also revealed that HALF of people in intensive care units were now Covid-19 patients.
But he said that at the moment the NHS is coping and will cope with an increase in cases.
The peak will come in the next seven to ten days before plateauing and hopefully starting to decrease, he confirmed.
That could be on Easter Sunday, the Health Secretary said last week.
But any lockdown measures are not likely to be lifted until after case numbers start to go down, he said.
“There is no point having gone through this effort and releasing the lockdown at a point where case numbers are still high and then will surge even faster than we’ve seen before,” he added.
“We want case numbers to get to a low point where we can start substituting other measures for the most intrusive and economically costly aspects of the current lockdown. ”
The lifting of measures will almost certainly include plans to massively ramped up testing – and going back to try and identify cases of transmission too.
Immunity certificates will be likely to be issued to people who have had the virus as well.
His comments came after photographs of people ignoring the rules and spending their weekends lying around sunning themselves emerged this weekend
And today the Heath Secretary came out to remind them they were risking lives.
He threatened to ban outdoor exercise unless people respected the rules.
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Mr Hancock told the BBC’s Andrew Marr Show today: “We’ve said because of the positive benefits to your physical and your mental health that it’s OK to exercise on your own or with members of your own household.
“But if the result of that is that too many people go out and flout the other rules because they say ‘well if I can exercise then it’s fine for me to do other things’ then I’m afraid we will have to take action.
“If you don’t want us to take the step to ban exercise of all forms outside the home, you have got to follow the rules.
“Let’s not have a minority spoiling it for everybody.”