Professor Jonathan Van-Tam has urged people to "be very careful" in the coming months as he warned of a "problematic" Christmas period.
England's deputy chief medical officer said Covid-19 case rates are falling, but he is concerned that an increase in deaths suggests the infection is "starting to penetrate into older age groups".
He said two factors will determine how the pandemic plays out over winter - how much caution people exercise over the coming months and the success of the vaccination programme.
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Asked how a Christmas lockdown can be prevented, Prof Van-Tam said: “Christmas, and indeed all of the darker winter months, are potentially going to be problematic."
He predicted that the UK would be "in a much calmer set of waters by spring", but warned that people should "be very careful", adding that booster jabs for the most vulnerable would be " really important" over winter.
Prof Van-Tam told BBC Breakfast that rates of Covid-19 are "still very high" and "higher than in most of Europe".
He said: "It’s of concern to scientists that we are running this hot this early in the autumn season.
“And so, from that perspective, I’m afraid it’s caution, followed by caution, and we need to watch these data very carefully indeed over the next days and weeks.”
Prof Van-Tam said that the number of deaths has seen an increase but hospital admissions have plateaued.
“Deaths are increasing – there might be some artefacts in the very latest figure – but essentially deaths are increasing,” he told the BBC.
“If you then look at hospital admissions, those have plateaued in the last four days. And if you look at the total number of patients in hospital with Covid, those have gone down in the last two or three days, but only a small bit.
“So what that tells me is that we have to just wait and see a bit longer – this could be a pause before things go up, it could be the very first signs that things are beginning to stabilise but at a high rate."
He said the drop in cases could be attributed to "the fact that this big wave we’ve had in teenagers is now starting to slip away".
Calling on people to get a booster jab if eligible, he said his main worry was that infection was spreading among older age groups leading to a higher number of deaths.
"That’s why the really key thing is that if you are called for your booster, if you are called for your flu vaccine, please go and get them," he said.
"This could be really very important this winter, it is not the time to be complacent.”
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Prof Van-Tam said he believes “too many people believe that this pandemic is now over".
“I personally feel there are some hard months to come in the winter and it’s not over," he warned.
“I think a whole range of behaviours, including the use of face coverings, but generally the caution that people take or don’t take in terms of interacting with each other – that is going to be a big determinant in what happens between now and the kind of darkest months are the winter.
“The other things that are going to be really important are how people respond if they are in need of a booster, if they are in need of flu vaccine, if they are partially vaccinated, or indeed if they are unvaccinated – that will be another really important factor in terms of what happens over the next few months.”
Asked about measures people should be taking in the lead up to Christmas, Prof Van-Tam said meeting outdoors or in well-ventilated areas would be "the most important factor", but masks should be worn "if others feel uncomfortable or if authorities ask a person to do so".
He said wearing face masks is useful “but they are probably most useful when used in combination with other things”.
The deputy chief medical officer said, in food and drink settings, face coverings were best to be worn as you’re entering and exiting the premises.
He said in cinemas and theatres “you’re not interacting with anyone in there other than the audience or the actors on the screen, you’re not interacting with the audience, you are seated quite close together, and for most people there should be relatively little personal expense involved in wearing a face covering in those kinds of settings.
“And then, finally, you’ve got the kind of very high mobility, high interaction venues such as clubs and here you can see it would be very socially inhibiting to wear face masks. I think if the epidemiology gets worse in the UK that’s on its own would not be enough in those kinds of venues.”
Asked whether people will need a booster every six months, Prof Van-Tam, said: “I think, on first principles, it’s quite likely that the third dose that you get, the booster dose, is likely to last a bit longer than the protection that we have from the first two doses, which is now starting to wane.
“But I can’t give you a straight answer to that because the truth of the matter is this is a new disease, we are still learning about it, we are still learning about how long these vaccines will last for after the third dose.
“And we have to wait for those data before we can give you absolutely emphatic answers.
“But is there some kind of known issue about having to have repeat doses of vaccine? Well, no, there isn’t, and you can see that from the annual flu jab which is given year after year, decade after decade to the elderly, to people with high-risk conditions.”
Meanwhile, Professor Jeremy Brown, who is a member of the JCVI, said that, while the current community infection rate is “high”, he does not believe the current case numbers of coronavirus are “out of control”.
He told BBC Radio 4’s Today programme: “The hospital admissions is significant and it’s bubbling under at the same rate, relatively speaking, over the past few weeks.
“We are at a relatively important crux point here where, if things start to settle down and infection rates in hospitalisations goes down, then that’s very encouraging.
“But it is its precarious point… if it starts going up and there’s a different situation completely.”
