The prime minister has warned coronavirus "is still out there" but vowed his new COVID winter plan "will give us the confidence" to avoid lockdowns.
Speaking at a Downing Street briefing, Boris Johnson said that higher case numbers than this time last year meant that in one sense the country's position was "actually more challenging".
But he added that the UK was in an "incomparably" better position to fight COVID-19 because of the success of its vaccination programme.
As a result, the PM said the government was "going to keep going" and would be "sticking with our strategy".
Asked what would have to happen to force the government to bring in harsher measures, chief medical officer Professor Chris Whitty said the arrival of a new variant that was both transmissible and "escapes the vaccines" would be one scenario where different rules could be introduced.
And he warned that the UK had not yet faced a winter with the "very bad" Delta variant.
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Mr Johnson did not get into specifics when asked how bad the situation with COVID would have to get for the government to start reimposing restrictions.
But he said the government's "plan B" has "a number of different shots in the locker" and "you wouldn't necessarily play them all at once".
"We're now in a situation when because so many of the population have some degree of immunity, smaller changes in the way we're asking people to behave can have a bigger impact," the PM said.
Asked why the government was not taking mild preventative measures now, Mr Johnson said vaccines were making a "huge difference to mortality".
"We are continuing to advise everybody to be sensible and responsible… I think that's the right balance, given where the pandemic is at the moment."
He added that the priority should remain the five million people who are yet to receive a vaccine.
Prof Whitty insisted "lots of people, probably the great majority" are taking measures to prevent the spread of the virus.
The prime minister also said that because such a large proportion of the population now had had immunity, "smaller changes can make a bigger difference".
He said: "COVID is still out there. The disease, sadly, still remains a risk."
But he added: "We are confident in the vaccines that have made such a difference to all of our lives."
Prof Whitty, also showed a series of graphs comparing case numbers, hospitalisations and deaths, this September and last September.
The briefing comes on the day the government set out its winter plan for managing COVID-19 in England.
"Plan A" for the coming months includes a booster jabs campaign which will begin next week.
People will also be encouraged to meet outdoors or open windows if inside, wear a face mask in crowded and enclosed places, wash their hands frequently and use the NHS COVID-19 app.
Businesses are also being urged to consider using the NHS COVID pass to check whether customers have been fully vaccinated or recently tested negative.
Speaking in the Commons earlier, Health Secretary Sajid Javid said it was "highly likely" that frontline health service staff and those in the wider social care sector would be required to have coronavirus and flu vaccinations.
He said contingency plans would only be activated if there was "unsustainable" pressure on the NHS and initial efforts to control the spread of the virus were ineffective.
Under the government's "plan B", mandatory face masks, vaccine certificates, and work from home orders could return.
Ministers have also not ruled out another lockdown, although they have stressed this would be deployed as a last resort and would require legislation to be passed.