Boris Johnson has set out his hope the current COVID lockdown will be the last thanks to the "unprecedented" vaccine rollout - but admitted he can't offer "an absolute cast-iron guarantee".
Speaking at a Downing Street news conference, the prime minister hailed the "astonishing efforts" of everyone involved in the delivery of jabs after the government met its target of offering the first dose to 15 million of the most vulnerable.
He praised the "unprecedented national achievement", but said that now was "no moment to relax" as the vaccine rollout moves on to the next five priority groups.
Mr Johnson reiterated his intention to set out, on 22 February, how England's third national lockdown will be eased and said the public "must be both optimistic but also patient."
"Next week I'll be setting out a roadmap saying as much as we possibly can about the route to normality, even though some things are very uncertain," he said.
"Because we want this lockdown to be the last and we want progress to be cautious but also irreversible.
"So please continue to stay at home, protect the NHS and save lives."
But the prime minister later said that, while he was "increasingly optimistic" about the possibility of easing restrictions, he couldn't guarantee the current lockdown would be the last ever.
"I'm very hopeful that we'll be able to go ahead and open things up, but to say I can give an absolute cast-iron guarantee that we won't face further difficulties and have to think harder and deeper about some problems, no at this stage I can't," he said.
Mr Johnson also urged people to take reports on how and when lockdown restrictions might be eased "with a pinch of salt".
"The data becomes clearer with every day that passes and we have to continue to evaluate," the prime minister said.
"And also, to be absolutely clear, these decisions we will take in the course of this week but they are not yet taken.
"And I would just humbly advise anybody reading accounts of what we are going to do... at such and such a time in the course of the next few months, to take them with a pinch of salt, folks because this is still speculation."
Sir Simon Stevens, the chief executive of NHS England, said that reaching the first target of the UK's vaccination programme was "definitely not mission accomplished".
"I've described the vaccination campaign as two sprints and a marathon - we've just come through the finishing line of the first sprint," he said.
Sir Simon described the vaccination of the next priority groups, including all over-50s, between now and the end of April as the "second sprint".
And the "marathon" effort would involve "vaccination for everybody in the country who's eligible, as well as the winter flu campaign and, potentially, further COVID booster shots this autumn".
Sir Simon also said the next phase of the UK's vaccination programme - for more than 14 million of the next most vulnerable - would see "pretty substantial ramp-up" in the number of jabs given.
"In this next phase, this second sprint, actually we're going to be vaccinating a larger number of people than in the first sprint," he added.
"And, overall, although supply will vary week to week and we'll have to adjust accordingly, we may be giving up to twice as many vaccinations overall - given we've got to be doing the second doses as well - then we've done in the first sprint."
England's chief medical officer, Professor Chris Whitty, said the COVID rates were "definitely heading in the right direction", but he highlighted how there are still more people currently in hospital with coronavirus than at the peak of last spring's first wave of infections.