Millions of people are facing travel disruption and increased Covid restrictions over Christmas, as the surging Omicron variant sees safety curbs tightened and flights cancelled.
Italy, Spain and Greece have made face masks compulsory outdoors again.
Catalonia, in northern Spain, has imposed an overnight curfew, and the Netherlands is in a strict lockdown.
Despite early findings that Omicron is milder than other variants, scientists are concerned by the number of cases.
Record infections were tallied in the UK, France and Italy on Thursday.
Meanwhile in the US, daily Omicron cases have risen beyond the peak of the recent Delta wave, and hospitals are filling up across the country.
"When we have millions and millions and millions of people, all sick, all together at one time, it doesn't take a large percentage of those people to topple over the hospitals," Dr Hallie Prescott, associate professor of internal medicine at the University of Michigan, told the New York Times.
America's top infectious disease expert, Dr Anthony Fauci, warned earlier this week that Christmas travel would increase the spread of the variant even among the fully vaccinated.
On Christmas Eve (Friday), US airlines said they were already suffering from staffing shortages due to flight crews testing positive or being forced to self-isolate.
United Airlines pulled about 150 flights scheduled for 24 December, according to the FlightAware website, citing the "nationwide spike in Omicron cases".
Delta Airlines cancelled 90 flights, and Alaska Airlines scrapped a further 17 after some employees "may have been exposed to the virus", AFP news agency reports.
