The British public has been warned that the coming months will be “challenging” as coronavirus cases reached almost 50,000, the highest daily level since July 17.
Downing Street said an increase in coronavirus cases had been expected over the winter and the government would keep a “close watch” on the situation.
Epidemiologist and government adviser Prof Andrew Hayward said the situation was “concerning” and there was “huge potential for the NHS to come under a lot of pressure”.
Government data up to Monday shows there have been another 49,156 lab-confirmed Covid-19 cases in the UK.
Another 45 people had died within 28 days of testing positive for Covid-19 as of Monday, bringing the UK total to 138,629.
Figures from the Office for National Statistics on Friday showed coronavirus infection levels in England are getting close to the peak of the second wave and are mostly being driven by rates among schoolchildren.
“We obviously keep very close watch on the latest statistics," Prime Minister Boris Johnson's spokesman said. “We always knew the coming months would be challenging.
“What we are seeing is case rates, hospitalisations and deaths still broadly in line with the modelling as set out a few months back now.
“The vaccination programme will continue to be our first line of defence, along with new treatments, testing and public health advice. But we will obviously keep a close watch on cases.
“But it is thanks to our vaccination programme that we are able to substantially break the link between cases, hospitalisations and deaths.”
The spokesman said the success of the vaccines meant “we are able to be one of the most open economies in Europe, which is benefitting the public and indeed businesses as well”.
“I think it’s concerning that we’ve got very high rates of infection and higher rates of hospitalisation and mortality than many of our European counterparts,” Prof Hayward, a member of the Sage scientific advisory panel, told BBC Radio 4.
He said waning immunity was “probably part of” the reason infections were high.
Prof Hayward said that there was “some evidence” that protection against infection was beginning to wear off, and “probably some evidence” that protection against severe disease was waning to a lesser extent.
“We shouldn’t be complacent because there is still huge potential for the NHS to come under a lot of pressure and for there to be a lot of unnecessary deaths," he said.
“So we need to get the vaccination rates up and we need to be prepared potentially to think about other measures if things do get out of control.”
“Different countries are potentially at different stages of their vaccination programmes and have different measures in place, so it’s difficult to compare and contrast," Downing Street said.
The Prime Minister’s spokesman said: “What’s important is we strike the right balance between protecting lives and livelihoods.”
The government’s autumn and winter plan suggested that some measures including the mandatory use of vaccine passports and face coverings could be required in England if cases were putting unsustainable pressure on the NHS.
But Downing Street insisted: “There is absolutely no plan to introduce Plan B currently."