Boris Johnson warned it would take time to ease lockdown restrictions in England which could be in place until the end of March.
The Prime Minister, who came under pressure from senior Tories to commit to easing the restrictions as soon as possible, said there would be “substantial opportunities” for relaxation before March.
But he warned there would not be a “big bang” where all the curbs on freedoms were removed at once.
His comments came as MPs prepared to retrospectively vote on the lockdown which came into effect on Wednesday.
The Prime Minister said there was “no choice” but to issue the stay-at-home order and close schools given the spread of coronavirus, particularly the more infectious new variant.
As a result of the measures – which will see the majority of pupils kept out of classrooms until at least after the February half-term – GCSE, AS and A-level examinations will once again be cancelled this summer.
They will be replaced by school assessments as ministers and regulators seek to avoid the chaos caused last year by the use of an algorithm to determine grades.
Addressing MPs after the recall of the Commons from its Christmas break, Mr Johnson said there was now a race between the spread of the virus and the delivery of vaccines to the most vulnerable.
Mr Johnson said: “Our emergence from the lockdown cocoon will not be a big bang but a gradual unwrapping.”
The regulations run until March 31 “not because we expect the full national lockdown to continue until then but to allow a steady, controlled and evidence-led move down through the tiers on a regional basis”.
The Prime Minister told MPs there would be “continuous review” of the measures with a statutory requirement to look at them every fortnight and a legal obligation to remove them if they are no longer necessary.
“We are in a tough final stretch, made only tougher by the new variant,” Mr Johnson said.
“After the marathon of last year we are indeed now in a sprint, a race to vaccinate the vulnerable faster than the virus can reach them.
“Every needle in every arm makes a difference.”
The Government is giving the vaccine to as many priority patients as possible with a second shot after 12 weeks, rather than holding back supplies to offer a booster dose three weeks after the first jab.
The World Health Organisation said it would not recommend that approach, instead suggesting the interval between doses should be between three and four weeks.
There are also concerns about the speed with which vaccines can go through the necessary safety checks to be deployed and Mr Johnson also faced questions about the bureaucracy which is making it harder for people to volunteer to help deliver the vaccines.
Seven mass vaccination will open next week with sites in London, Newcastle, Manchester, Birmingham, Bristol, Surrey and Stevenage.
Vaccines minister Nadhim Zahawi admitted the target of vaccinating around 14 million people in the highest priority groups – including the elderly, those with clinical needs, care home residents and staff and frontline NHS workers – by February 15 was “stretching”.
Some 1.3 million people have already received either the Pfizer/BioNTech or Oxford/AstraZeneca jab.
But Mr Johnson confirmed that just 10% of care home residents had received a vaccine, along with 14% of staff.
The Government wants England’s schools to reopen after the February half-term, but experts warned that would depend on progress in the battle to curb Covid-19 cases.
Former chief whip Mark Harper, chairman of the Covid Recovery Group (CRG) of lockdown sceptics, used a Telegraph article to call for a “substantial relaxation” of restrictions as soon as the four top priority groups have been vaccinated.
Fellow CRG member Steve Baker said: “Once the most vulnerable have been vaccinated, draconian restrictions must be substantially removed.”
Sir Graham Brady, chairman of the influential 1922 Committee of backbench Tories, called for Commons votes at the end of January and February on whether the restrictions should continue.
In other developments:
– The Metropolitan Police indicated a tougher approach to those flouting the lockdown with officers expected to be more “inquisitive” about why people are “out and about”.
– The European Medicines Agency cleared the way for the Moderna vaccine to be used in the European Union, but following the end of the Brexit transition period on December 31 this will not apply in the UK.
– All scheduled GCSE and A-level examinations in Northern Ireland will be cancelled.
– Dr Andrew Goodall, chief executive of NHS Wales, said critical care capacity is operating at almost 140% of its normal occupancy.
Labour leader Sir Keir Starmer backed the lockdown , saying: “The virus is out of control, over a million people in England now have Covid, the number of hospital admissions is rising, tragically so are the numbers of people dying.”
But he added “this is not just bad luck, it’s not inevitable, it follows a pattern” of the Government being slow to respond.
Graham Medley, professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine, and a member of the Scientific Advisory Group for Emergencies (Sage), said “we’re in for a long haul” in the fight with coronavirus.
He told BBC Radio 4’s Today programme that measures to curb the spread of coronavirus could be needed “next winter or even the winter after”.