He told broadcasters the country was in a “much, much stronger position than this time last year… because of science and because of vaccination”, but he warned against complacency.
It would be “absolute folly” to think the coronavirus pandemic was “all over”, Mr Johnson said.
He said: “I would say to everybody looking at the pressures on the NHS in the next couple of weeks, and maybe longer, looking at the numbers of people who are going to be going into hospital, it will be absolute folly to say that this thing is all over now bar the shouting.
“We’ve got to remain cautious. We got to stick with Plan B. We’ve got to get boosted.”
Asked whether resisting further restrictions would risk putting further strain on the NHS, Mr Johnson said the current measures are the right ones to address the virus.
Pressure on the NHS for the “next couple of weeks and maybe more” is going to be “considerable”, Mr Johnson has said.
The Prime Minister said the Omicron variant of coronavirus is “plainly milder” than other strains.
But he said “there’s no question Omicron continues to surge through the country”.
He added: “I think we’ve got to recognise that the pressure on our NHS, on our hospitals, is going to be considerable in the course of the next couple of weeks, and maybe more.”
Mr Johnson said the booster programme and Plan B measures made a difference, but added: “There are still quite a lot of people who have had two jabs, but haven’t had the third. The third jab really does make a big, big difference.”
He added that all measures will be kept under review and urged people to “build up defences” by getting third doses of the jab.
People should also take Covid tests “before meeting people you don’t usually meet” and work from home if possible, the Prime Minister said.
He said: “I think the way forward for the country as a whole is to continue with the path that we’re on. We’ll will keep everything under review.
“The mixture of things that we’re doing at the moment is, I think, the right one.
“So, number one, continue with Plan B, make sure that people take it seriously, do what we can to stop the spread, use the Plan B measures, work from home if you can, wear a mask on public transport… take a test before going out to meet people you don’t normally meet, think about the the requirements under Plan B, but also get the boost.”