The British Medical Association said cancelling patients booked in for their second doses was “grossly unfair”.
Gavin Williamson said he wanted school closures to be as “short as possible” after delaying their reopening amid surging coronavirus cases.
Earlier, the Oxford-AstraZeneca vaccine was approved for use in the UK, with the first doses to be given on Monday.
It comes as millions more people in England are expected to be placed under the toughest tier four restrictions.
The whole Scottish mainland is in the highest level of restrictions in a bid to contain a new strain of the virus.
NHS England’s chief executive Simon Stevens said health workers were “back in the eye of the storm”.
Paramedics in London are now receiving almost 8,000 call-outs a day, with Boxing Day described as one of London Ambulance Service’s busiest in history.
The diplomats will meet to determine how they can provisionally approve the deal, which was agreed on Christmas Eve, covering £660bn of trade in time for the end of the transition period.
The UK reported another 30,501 positive tests on Sunday, and 316 deaths of people who had tested positive within the past 28 days.
Boris Johnson told the Sunday Telegraph the deal would provide new legislative and regulatory freedoms to “deliver for people who felt left behind”.
The details given a week ago of the new variant in England triggered travel curbs from dozens of countries.
It comes after official UK coronavirus deaths passed 70,000 on Christmas Day.
Brexit trade deal is done, says UK government UK and EU reach post-Brexit trade deal ending months of arguments over business rules and fishing rights.
Amid speculation that both sides were close to unveiling a deal on Wednesday evening, European Commission spokesman Eric Mamer tweeted just after midnight GMT to confirm the talks would be continuing.