The prime minister has admitted "we don't know... how strong our fortifications now are" against a fresh COVID wave - while the chief medical officer has warned of a "leaky wall" of vaccine defence.
Speaking at a Downing Street news conference on Monday - as people in England saw lockdown restrictions eased slightly - Boris Johnson urged the need for caution as COVID rules are lifted.
People across England are now allowed to meet in groups of up to six or as two households outdoors, with outdoor team sports also able to resume.
And while the prime minister acknowledged a "big day" for many people to see friends and family after a number of months, he also said the country "must proceed with caution".
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"That wave is still rising across the Channel and it is inevitable as we advance on this roadmap that there will be more infections, and unavoidably more hospitalisations and sadly more deaths," he said.
A recent surge in COVID cases in countries such as France and Germany has brought the imposition of new restrictions across Europe.
Although the UK has been faster in rolling out COVID vaccines than other European countries - with more than 30 million people in the UK now having received a first dose - Mr Johnson said it was unclear how "robust" the vaccine programme would prove against a new spike in infections.
"What we don't know is exactly how strong our fortifications now are, how robust our defences are against another wave," he said.
"We have seen what is happening with our European friends. Historically, at least there has been a time lag and then we have had a wave ourselves.
"That's why I stress the importance of everybody maintaining the discipline people have shown for so long."
Professor Chris Whitty, England's chief medical officer, said the UK had a "kind of wall of vaccination that will get stronger" as more people begin to receive their second doses over the coming weeks.
But he added: "It is not a complete wall, it is a kind of leaky wall. Therefore, there will always be some people who either have chosen not to be vaccinated, or where the vaccine has had much less effect.
"If we get a small surge, there will be cases of people who have been vaccinated who will have severe disease, and there will be cases of people who are not vaccinated, a much higher proportion, who will get severe disease, and some of those will go on to die.
"If you get a very big wave, that would obviously lead to a significant impact.
"So that's the reason why the prime minister and ministers have been absolutely determined that this is a slow and steady unlocking, looking at data between each step."
Prof Whitty also advised grandparents not to hug their grandchildren this Easter, even if they have had both vaccine doses.
"The thing to understand with vaccines is they provide increasing levels of protection as we go through," he said.
"The first vaccine provides a high degree of protection, the second vaccine for the same person provides greater protection - but there's still some vulnerability.
"Then actually having people around someone who has been vaccinated who are themselves vaccinated, provides a further level of protection.
"And then the key thing is keeping the rates right down, which makes it very unlikely that someone who comes in - even if they haven't been vaccinated and certainly if they have - will actually have COVID and be able to pass it on.
"What we're trying to do is get to the point where all of those protections are in place. We are not yet at that stage. We are getting there steadily."
Prof Whitty also urged people to remember they are "not immediately protected" when they receive a COVID jab.