Senior British politicians have demanded teachers be made a higher priority for Covid-19 vaccinations.
Prime Minister Boris Johnson is under pressure to delay the reopening of schools in England, where admissions to government hospitals exceeded the peak of 19,000 recorded in April.
“Teachers and parents need a clear and definitive statement that schools will not be required to reopen in January until effective vaccination is made available to teaching staff,” said Conservative party MP Roger Gale.
“Education is important but so are the lives and wellbeing of teachers.”
The over-80s, health workers, and care home residents and staff are the only groups currently eligible to receive the Pfizer/BioNTech vaccine in the UK.
The vaccine will later be offered to all people aged over 50, but there is no plan to offer it to younger adults unless they have serious conditions that make them especially at risk.
Scientists said they fear a "catastrophe" in the coming months if restrictions are not tightened before schools reopen.
"From a purely epidemiological point of view it makes a lot of sense to keep schools closed for longer and introduce more stringent tests in them," Prof Andrew Hayward, a scientific adviser to the government, told BBC Radio 4's Today programme.
“We’re going to have to get schools back – maybe a little bit later – but we’re going to have to have increased restrictions in other areas of society to pay for that.
“We are entering a very dangerous new phase of the pandemic and we’re going to need decisive, early, national action to prevent a catastrophe in January and February.
“A 50 per cent increase in transmissibility means that the previous levels of restrictions that worked before won’t work now, and so Tier 4 restrictions are likely to be necessary or even higher than that. I think we’re really looking at a situation where we’re moving into near lockdown,” he said.
Ministers said teenagers sitting public exams in England in 2021 will return on January 4, with other children resuming face-to-face lessons later in the month.
Welsh and Scottish schools have delayed or revised the start of the spring term, while in Northern Ireland, schools are expected to reopen on January 4.
The UK government on Tuesday pledged to draft in the armed forces to introduce coronavirus testing in schools, but the announcement did little to quell the fears of a growing number of trade unions, politicians and scientists who doubt the system will be ready in time.
Teachers have expressed concern for their own health if they have to supervise pupils taking swab tests, who will be expected to administer them on themselves.
Steve Chalke, the founder of the Oasis Academy Trust chain of 52 secondary schools, called for a short delay to make sure the system is effective.
“We would ask government to pause, to come up with a clear strategy for the continuity of education,” he said.
“If we’re reckless we will all pay and these children will pay later.”
The GCSE and A-level exams are taken in years 11 and 13, respectively, when most candidates are aged 16 and 18.
Since the fallout from the decision to close schools in March, which led to the cancellation of exams and a furore over university admissions in the summer, ministers have repeatedly said education must continue even if other parts of society and the economy have to close to accommodate it.
Education Secretary Gavin Williamson is said to be concerned for vulnerable children and their long-term prospects if schools do not reopen, The Times reported.
Mr Johnson has also made keeping schools open a key priority as he looks for ways to kick-start the economy after months of restrictions left it facing its worst downturn for 300 years.
Ministers threatened legal action to stop schools offering home learning before Christmas, but a government statement late on Monday left open the possibility of that position being reversed in the New Year.
“We want all pupils to return in January as school is the best place for their development and mental health,” it said. “But as the prime minister has said, it is right that we follow the path of the pandemic and keep our approach under constant review.”
On Monday the UK’s daily case tally surged to a record 41,000.