(Bloomberg) -- U.K. Prime Minister Boris Johnson left the door open to another coronavirus lockdown this winter, warning that people must get their Covid-19 vaccinations and booster doses to avoid fresh restrictions.

A “new wave” of Covid-19 is spreading across Europe and forcing governments to reimpose tougher rules, Johnson said at a televised press conference on Monday. “History shows we cannot afford to be complacent.”

U.K. Expands Covid-19 Booster Program to Younger People 

Asked directly about the possibility of another lockdown this Christmas, Johnson replied that there is nothing in the current data that signaled the need for restrictions, but warned: “clearly we cannot rule anything out.”

Any move to reintroduce pandemic restrictions would likely be politically perilous for Johnson, who imposed an effective lockdown at the last minute over Christmas last year. Yet his comments are a marked change in tone from his insistence last month that there will be no curbs over the holiday season.

England’s chief medical officer Chris Whitty, standing alongside Johnson, said U.K. coronavirus cases and hospitalizations had been “broadly flat” for some weeks. But he said large parts of the National Health Service are under “very significant pressure” and likely to remain so over the winter.

Earlier on Monday, the U.K. government said it is expanding its coronavirus vaccine booster program to people in their 40s as it tries to head off another wave of infections.

But while Johnson warned of the risk from Europe, some scientists argue the U.K. -- which has seen a high daily caseload since July -- is actually ahead of the most recent wave sweeping the continent.

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