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Boris Johnson delayed the end of lockdown until 19 July after being warned unlocking on Monday could have heaped unsustainable pressure on the NHS and led to thousands more coronavirus deaths.
The Delta variant of the virus, first identified in India, has changed the outlook of the pandemic.
Here are five charts, shared by England's chief medical officer Prof Chris Whitty, that demonstrate how serious the situation is right now and why a delay to ending restrictions was deemed necessary.
1. More second vaccine doses needed
While one dose of a vaccine leads to an estimated 57% to 85% reduction in Delta variant hospital admissions, it provides much less protection – a 26% to 40% reduction – for symptomatic disease.
"You do need two doses of the vaccine to provide a strong degree of protection," Prof Whitty said at Monday's Downing Street press conference.
As of Sunday, 43.1% of UK adults – more than 20 million people – still needed to receive a second dose and therefore would have been at higher risk of symptomatic COVID if restrictions had been lifted on Monday.
2. 'Rapid rise' in infections
Infections in both the North West, the region where case numbers are highest, and England as a whole have increased 64% in a week.
Prof Whitty explained: "The rates of hospitalisations at the moment are low in all parts of the country but – and this is the important ‘but’ on this test – what we are seeing is a rapid rise [in infections], particularly in the North West of the country.
"But the rest of the country is following."
Watch: Lockdown restrictions – what is changing, what is staying the same and why?
The now-dominant Delta variant is said to be 60% more transmissible than the previously dominant Alpha variant.
Prof Whitty went on: "A relatively small number of doubling times and you start getting into really quite large numbers. If you’re doubling from 1,000 to 2,000 to 4,000 in less than two weeks, clearly things can take off very rapidly."
3. Cases increasing across whole country
To put it simply, COVID-19 cases are spreading across the whole country because of the Delta variant.
The map above highlights widespread "brown" areas where infection numbers are increasing, with "grey" areas of decreasing cases notably sparse.
4. NHS 'would run into trouble'
“The link between people getting an infection and being hospitalised has been substantially weakened," Prof Whitty said.
"A much smaller proportion of those infected are going into hospital – but it has not been completely stopped."
With a 50% increase in hospital admissions in England in a week, Prof Whitty said this would "probably accelerate" given the increased case numbers.
“Although we do not think an immediate overwhelming of the NHS is likely," he went on, "if this continues on an exponential path and if that accelerated further [due to lockdown being lifted on 21 June] then we would run into trouble in a relatively small number of doubling times.”
5. Delta variant rising 'exponentially'
Prof Whitty said that "while the Alpha variant is going down, the Delta variant is going up exponentially" and "this is the reason why the assessment of risk has fundamentally shifted".
He warned infection numbers will be even higher by the new unlocking target date of 19 July. Crucially, however, the continuation of social distancing and more double jabs should prevent a "high peak".
Prof Whitty said: "At the end of the period the prime minister is talking about, we will have a situation where a much higher proportion of people have been double-vaccinated coming up to school holidays.
"We all want to stress that at that stage, rates will be higher than they are at the moment. But the next four weeks will reduce significantly the risk of a very high peak which could cause significant problems in terms of pressure on the NHS, the knock-on effects of that, and direct deaths from COVID."
Watch: Tuesday's daily politics briefing