The UK has hit a "dangerous moment" in the pandemic, an expert has claimed, amid warnings that "vaccine-beating variants" will emerge in the future.
Dr David Nabarro, the World Health Organisation's special envoy on COVID-19, said that despite low infection levels, unlocking restrictions too quickly in the UK could create a further "spike" in infections.
He also issued a warning about variants, saying "vaccine-beating" forms of the virus will almost certainly emerge sooner or later.
Referring to the UK's vaccine success, he told Sky News: "It is always the most dangerous moment when you are managing an infectious disease as you get that curve coming right down to zero.
"It's at that point that if you release your precautions too quickly then a spike will come back up."
He added: "And given that we've got new variants, particularly variant Delta (the Indian variant) in the UK, we have to be doubly careful.
"So my only suggestion to everybody is please continue to treat this virus with respect.
"It hasn't actually become any less dangerous - so maintain the physical distancing and mask-wearing as much as you possibly can. There's nothing to be gained from perceiving that bravado will somehow beat the virus. It doesn't work like that."
He was echoed by another expert who said the country could be "slammed" by new coronavirus variants unless more is done to help poorer nations vaccinate their populations.
Although vaccines have been effective against the Delta variant first identified in India, protection against future variants is "not guaranteed forever", Sir John Bell, professor of medicine at Oxford University and a member of the Oxford/AstraZeneca team, said.
He told BBC Radio 4's Today programme: "This disease is here to stay, probably forever, but we do need to move on and try and make sure we suppress the diseases as much as possible around the world because otherwise we're just going to sit here and get slammed by repeated variants that come in the door."
Asked if his words of caution meant he was in favour of delaying England's 21 June unlocking, Dr Nabarro replied: "I'm perfectly okay if people are going to say to me the government's going to relax restrictions, but what I want to add is, could we also have a really strong emphasis on partnership between people and government and business because we are going to have to continue to be vigilant and to be able to deal with this virus for some time to come.
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"Right now, I don't think we have any vaccine-beating variants on the horizon, but that will almost certainly happen sooner or later.
"So, let's be COVID-ready to deal with spikes when they come and stop them turning into surges."
Boris Johnson said on Wednesday that he still sees nothing in the data to suggest the plan to end all legal lockdown restrictions in England on June 21 will need to be delayed.
Yet he stressed a need to be "cautious" due to "ambiguous" data on how successful the UK's vaccine programme would be in combatting a new surge in infections.
However, some academics have said the date needs to be delayed.
Professor Adam Finn, of the Joint Committee on Vaccination and Immunisation (JCVI), said there are still many people who are vulnerable to the effects of COVID-19, as he warned "the idea that somehow the job is done is wrong".
Professor Ravi Gupta, a member of the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said a delay of a few weeks could have a significant impact on Britain's battle against the pandemic and recommended it should be made clear to the public that it would be a temporary measure based on the surge in cases of the new variant.
Sir John told the BBC that current figures "don't look too intimidating" but they still need to "play out for a couple of weeks" before the government makes its final decision.
"We do need to keep our eye on hospitalisations, serious disease and death, which is really what we're trying to manage," he added.
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