England will see another "surge" of Covid cases this summer, autumn or winter, the Chief Medical Officer predicted today.
Chris Whitty said "all the modelling" suggests cases will soar at some point after restrictions ease - despite more than a third of the UK getting a first vaccine dose so far.
And he rejected calls to ease lockdown faster, telling MPs: "If you open up too fast, a lot more people die - a lot more people die."
But he added the "ratio of cases to deaths will go right down" compared to previous waves of the virus, due to the vaccine kicking in.
It comes after modelling for SAGE last month projected there would be more than 30,000 extra deaths by summer 2022 under all scenarios of easing lockdown.
Chief Scientific Advisor Sir Patrick Vallance said reopening schools this week could push the R number up by between 10% and 50%.
And he warned not even he and Prof Whitty know if the roadmap - which proposes opening beer gardens, non-essential shops, gyms and hairdressers from April 12 (Step 2), indoor gatherings from May 17 (Step 3) and ending all restrictions from June 21 (Step 4) - will be delayed.
Sir Patrick said politicians have to decide as they go along if they will be sticking to the dates or "delaying if you have to." But he warned politicians will be "flying blind" if they leave a gap of less than five weeks between each step, to analyse the data.
He told the Commons Science and Technology Committee: "Nobody would say we know exactly how this is going to roll out over the next few months.
"The important thing is to measure, adapt and take decisions in the light of information as it emerges.”
Prof Whitty told MPs: “As things are opening up, what all the modelling suggests is at some point we will get a surge in virus.
"Whether that happens, we hope it doesn’t happen soon, but it might for example happen later in the summer if we open up gradually, or if there’s a seasonal effect it might happen over the next autumn or winter.
“But I think all the modelling suggests there is going to be a further surge and that will find the people who have either not been vaccinated or where the vaccine’s not worked.
"Some of them will end up in hospital and sadly some of them will go on to die. That’s just the reality of where we are.”
The Chief Medical Officer said most people driving the surge in transmission are younger and will not be vaccinated by Easter.
Even as the vaccine rolls out, the “ratio of cases to deaths will go right down as a result of vaccination, but not right down to zero unfortunately”, Prof Whitty said.
Prof Whitty sent a powerful warning shot to Tory MPs calling for lockdown to lift quicker - and appeared to suggest Boris Johnson may regret not locking down more quickly.
He told MPs: "If you look at the history of this all around the world, the history of this is not full of countries and individual leaders wishing they had done more, faster. It's full of leaders who wished they had acted quicker and then been more careful as they take things off."
Prof Whitty said the lockdown easing proposals for April and May - Steps 2 and 3 - are a "very substantial block of things with very high risk when you put them together".
“A lot of people may think this is all over," Prof Whitty said.
"I would encourage them to look at what is happening in continental Europe at the moment, where a lot of countries are going back into rates going up and having to close things down again, having not been in that situation before.
"It’s very easy to forget quite how quickly things can turn bad if you don’t keep a very close eye on it."
It comes after Step One of Boris Johnson's roadmap out of lockdown began on Monday, with all schools in England returning at once and two people from two households allowed to meet for a picnic in the park.
Step One will continue on March 29 with up to six people or two households able to meet outdoors, including to eat and drink.
Tory MPs have urged the government to go faster, but Sir Patrick said: "We don’t know what the impact of, for example, schools going back is going to be.
“There’s an estimate from the modelling group that it could have an effect on R between 10 and 50% increase. We don’t know within that range exactly what it will be.
“We know that as contact increases transmission increases and we know that as schools go back other things happen as well - parents meet, people who perhaps weren’t going into work, some of them may go into work as a result. So it changes a number of things, all of which may have pressure on transmission.”
Sir Patrick however said replacing the 'big bang' opening of schools with a phased reopening - as demanded by unions - would ultimately have led to the same result.
The Chief Scientific Advisor said despite the government's aim to ease all legal restrictions from June 21 at the earliest, masks and keeping windows open should become a normal part of everyday life.
Sir Patrick told MPs the nation should be "making sure people understand that outside is less risky than inside, that maintaining hand hygiene is important, making sure we wear our masks in crowded places, absolutely agree that testing is important - and not just the testing, it’s the self-isolation that goes along with that.
“All of those are incredibly important ways in which we need to think about controlling this.
"And some of those need to become ways in which we behave just as a routine part of life. The more we can make this as a routine around hygiene matters, ventilation and so on, the better this is."
Some 4,712 people in the UK tested positive in the most recent 24 hours, a sharp fall, and case rates are below 100 per 100,000 in every region. Daily deaths also fell below 100 over the weekend.
But Deputy Chief Medical Officer Jenny Harries said on Monday the level - the same as it was at the end of September - is one from which “a new wave could easily take off again”.
