Professor Chris Whitty has warned a new Covid strain could be worse than Omicron as Boris Johnson cautioned Britain cannot "declare victory" over the virus yet.
Speaking at a Downing Street press conference, England's Chief Medical Officer said a new variant could also be "more vaccine-escaping" than the current dominant strain.
Speaking after he announced all remaining Covid restrictions will be axed, the PM said there "may be significant resurgences" and "it's very possible that those will be worse than Omicron".
Prof Whitty said that new strains were to be expected and while some "will just disappear" others could escape vaccines and result in a fresh wave in hospitalisations.
It comes after the Prime Minister revealed his Living With Covid strategy for England, which will see the end of free testing for all but the most vulnerable from April 1 - with a 72-hour limit already in place to stop stockpiling.
Mandatory self-isolation will also be axed from Thursday and the £500 isolation payments and day-one sick pay will go.
Prof Whitty said: “Some of those new variants will just disappear, but some of them will cause us significant problems and they could be either more vaccine escaping but as severe as Omicron, but the net effect would be actually more people end up in hospital because a lot of our protection is from vaccination, or it could be more intrinsically severe, because Omicron came from a much earlier variant.”
He added we “could certainly end up with something which is more likely to lead to hospitalisations than Omicron”, adding that winters are expected to be “tricky” even in the absence of significant new variants due to the combination of Covid, flu and other respiratory problems.
The Chief Medical Officer also urged Brits who test positive for Covid to self-isolate, even though they will no longer be forced to do so under law.
“As we look at the next weeks, we still have high rates of Omicron and I would urge people in terms of public health advice, and this is very much the Government’s position, that people should still if they have Covid try to prevent other people getting it and that means self-isolating," he said.
The PM told the press conference: "Today is not the day we can declare victory over Covid because this virus is not going away.
"But it is the day when all the efforts of the last two years finally enabled us to protect ourselves whilst restoring our liberties in full.
"And after two of the darkest, grimmest years in our peacetime history, I do believe this is a moment of pride for our nation, and a source of hope for all that we can achieve in the years to come."
He admitted, however, that there will likely be another variant that will “cause us trouble” but said he didn't want the public to think "there’s some division between the gung ho politicians and the cautious, anxious scientists".
He said: “We have a very clear view of this. This has not gone away. We’re able to make these changes now because of the vaccines and the high level of immunity and all the other considerations about Omicron that you’ve seen.
“But we have to face the fact that there could be, likely will be, another variant that will cause us trouble.
Prof Whitty said maximising ventilation, hand washing and using face masks in enclosed spaces with large numbers of people when there are significant numbers of cases remain important.
He also suggested he will keep taking lateral flow tests if going to a crowded place or seeing an elderly relative - even if he has to pay.
He told the Mirror “There are three things I will be trying to think about. The first question is how common is Covid at the moment? If it’s very high then absolutely, that increases the probability.
“Second is the environment you’re in - if you’re in an environment where you’re outside, a long way from other people, then the risks are so small it probably makes no difference. If you’re in a crowded environment, absolutely.
“And the third and probably the most important one… is if I’m mixing with vulnerable people. If I was going to see an elderly relative, if I was going to a haematology ward in a hospital. Of course those are the kinds of situations I would want to minimise the risk to other people. Because the whole point of the lateral flow is it’s about minimising the risk to other people.”
Asked if he was satisfied the plan would not lead to a rise in long Covid, and immunosuppressed people would be kept safe enough Chief Scientific Officer Sir Patrick Vallance told the Mirror: “I’d like the prevalence to be lower, the rates of infections to be lower.
“And the SAGE advice… shows that roughly somewhere between 20 and 40% reduction in transmission because of our current behaviours. So we’re not back to pre-pandemic behaviours here. And as those return, you would expect the pressure on transmission to increase.”
But he added: “There’s now evidence that the vaccines don’t just protect against infection and severe disease, but they also reduce Long Covid.”
Prof Vallance also said Covid will continue to evolve over the next couple of years and there was there was no guarantee that future variants would be less severe, adding: "You can be celebrating when the sun's shining, but take your umbrella with you."
He said it was essential to maintain a virus surveillance system, the capacity to “ramp up” measures again quickly and to protect the vulnerable.
“This pandemic is not over. The virus is continuing to evolve. It will continue to do so quite fast probably for the next couple of years,” he said.
“There is no guarantee that the next variant is as reduced severity as Omicron. As is it evolves what it is trying to do is to transmit more readily.
“The change in severity is a random by-product. We expect there to be further variants and they could be more severe.”
