Britons have been warned the coming months will be “challenging” as coronavirus cases reached the highest daily level since mid-July.
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 Professor 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 a further 49,156 lab-confirmed Covid-19 cases in the UK. This is the highest number of daily reported cases since July 17.
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 seen at the height of the second wave and are mostly being driven by rates among schoolchildren.
The Prime Minister’s official spokesman said: “We obviously keep very close watch on the latest statistics.
Watch: COVID-19 - UK daily coronavirus cases highest since July as Professor Chris Whitty warns of 'exceptionally difficult' winter for NHS
“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 benefiting the public and indeed businesses as well”.
Prof Hayward, a member of the Sage scientific advisory panel, told BBC Radio 4’s World At One: “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.”
He said waning immunity is “probably part of” the reason infections are currently high, adding there is “some evidence” protection against infection is beginning to wear off and “probably some evidence” protection against severe disease is waning to a lesser extent.
Prof Hayward added: “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.
“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.”
Downing Street said “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”.
The Prime Minister’s spokesman added: “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”.
ONS figures suggest that around one in 10 schoolchildren in Years Seven to 11 in England was estimated to have Covid in the previous week, the highest positivity rate for any age group.
But analysis by the PA news agency suggested there was low take-up of Covid-19 jabs among 12-15 year-olds.
In some areas the rate of vaccine uptake is as low as 5%, while only 15 local authorities in England have managed to give a first jab to at least a quarter of 12 to 15-year-olds, data shows.
The picture is very different in Scotland, where young people can also receive doses of the jab in drop-in vaccination centres, as the take-up is already over 50% in half of local authority areas.
There have been calls for vaccines to be offered to under-16s in walk-in centres rather than in school in order to boost take-up.
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James Bowen, director of policy for school leaders’ union NAHT, said: “Allowing 12-15 year olds to attend walk-in vaccination centres would be a sensible decision.”
The Prime Minister’s spokesman said: “In the first instance we are allowing for the jabs to take place through the school immunisation services, this is the long-standing approach that has been used for flu and HPV jabs.
“We are working very closely with schools, we are going to keep the programme for 12 to 15-year-olds under review.”
The spokesman said a number of factors could be behind the problems with getting jabs into children’s arms, potentially including “abhorrent” abuse and misinformation from protesters at school gates.
“It is completely unacceptable for anyone to direct abuse or misinformation towards parents, teachers or indeed children,” the spokesman said.
Meanwhile, French pharmaceutical firm Valneva, whose UK contract for vaccines was cancelled last month, has reported positive results from its Covid-19 trial.