The UK will face another "large wave" of coronavirus infections if all lockdown restrictions are lifted by late April and some measures will be needed beyond 2021, government scientists have warned.
Sir Patrick Vallance, the government's chief scientific adviser, said there is a risk of "flying blind" if all restrictions are scrapped at once because it takes about four weeks to measure the effect on COVID-19 cases of lifting a measure.
It comes after a new Public Health England report found among the over-80s that a single shot of the Pfizer vaccine cuts the chance of hospital admission and death from COVID-19 by more than 75%.
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A separate study involving 23,000 health workers in England also found those who received a single dose of the Pfizer jab had at least a 70% lower chance of becoming infected 21 days after vaccination, rising to 85% after a second dose.
In a briefing to journalists ahead of the prime minister's announcement on the "roadmap" to lift England's lockdown, Sir Patrick said that while vaccines are expected to make "a big difference", there are still uncertainties around vaccine efficacy; the proportion of the population that will have the jab; the level of restrictions needed longer term and whether the virus will be seasonal.
"Even with high vaccine levels, and indeed quite high vaccine coverage, it's important to remember a large number of people in the population remain unprotected," Sir Patrick said.
"It's likely you get an increase in cases when you start to open up - exactly when that occurs and exactly how high the numbers are, it's not possible to be precise.
"The sooner you open up everything, the higher the risk of a bigger resurgence. The slower you do it, the better."
Professor Angela McLean, the government's deputy chief scientific adviser, said in a scenario with "very fast unlocking", where most restrictions were lifted by late April, "it's pretty clear… that we would expect a large wave".
"It's common sense why that happens," she added.
"There remain people who have either not been vaccinated or even though these vaccines are absolutely fantastic, they are not perfect.
"There are people who have been vaccinated who are nevertheless not protected from severe disease.
"If we let a big epidemic happen amongst young people, some of those older, more vulnerable people - or vulnerable for other reasons - will get infected and will be very ill."
Senior Tory MP Mark Harper, who heads the COVID Recovery Group of lockdown-sceptic backbenchers, has said all legal restrictions put in place in response to the pandemic should be lifted by the end of April.
By that time, the top nine priority groups - including all over-50s - should have been offered a first dose of COVID-19 vaccine.
But in Monday's briefing, Sir Patrick said: "You really don't know what effect these unlocking measures will have until you've done them.
"The soonest you can really understand the effect is probably about four weeks.
"So you will be flying blind on this if you don't wait."
The government's scientific advisers have considered a document produced by the Imperial College COVID-19 Response Team - including Professor Neil Ferguson - which modelled the effect of different scenarios for lifting lockdown restrictions.
In the document, called "Unlocking" Roadmap Scenarios for England, the team warns that "vaccination alone will not be sufficient to keep the epidemic under control" due to "eligibility and vaccine hesitancy".
"Non-pharmaceutical interventions (NPIs) must be lifted slowly and cautiously to minimise the number of deaths and prevent high hospital occupancy, with some baseline NPIs remaining in place (and adhered to) throughout 2021 and beyond," the experts say.
"Relaxing too quickly… will result in peak hospital occupancy considerably higher than the current wave and substantial additional deaths.
"This holds regardless of vaccine efficacy, roll out, adherence to baseline NPIs, and impact of seasonality."
The Imperial College team said even a "gradual" lifting of restrictions - with primary schools returning on 8 March before all schools on 5 April and restrictions being eased over several months before being fully lifted on 2 August - there would be a "substantial additional number of deaths" with an estimated 58,200 fatalities by June 2022.