Sweden will require visitors from other Nordic nations to have a vaccine pass to cross the border as it tightens restrictions in the face of rising number of COVID-19 infections and worries about the Omicron variant, the government said on Thursday.

Sweden has seen new infections jump in recent days, if from levels below most European countries. It has reintroduced a limited number of measures and authorities said further steps would be needed if infections kept rising.

“The new virus variant Omicron makes it hard to predict how things will develop,” Health Minister Lena Hallengren told a news conference. “In Sweden, we are in a relatively better position, but even here infections and the burden on the health care system is increasing.”

The extension of vaccine pass rules to cover Nordic countries, hitherto the only nations not covered by the requirement, will come into force on Dec. 21.

Authorities also warned tighter restrictions may well be announced next week when the Public Health Agency publishes an update on how it sees the pandemic developing ahead.

Sweden reported 4,022 new cases on Thursday, lower than the day before but among the highest daily increases since late spring. Sweden, with some 10 million inhabitants, now has over 500 patients requiring care for COVID-19, with 79 of them in intensive care.

“We have an increase in cases and it has also led to a somewhat higher burden on the health care,” Health Agency official Britta Bjorkholm told a news conference, adding that cases were expected to keep rising.

Sweden has 84 confirmed cases of the Omicron variant, but Bjorkholm said the real number was likely to be higher as sequencing can take up to two weeks.

Despite the rise in cases, the situation in Sweden has had some scientists scratching their heads. Neighbouring Denmark, with a population roughly half that of Sweden reported 10,000 new cases on Thursday.

Sweden’s centre-left government has laid out a three-tier road map for its response to an increase in infections.

Norway has tightened restrictions to limit the spread of the Omicron variant amid record infections and hospitalisations.

Denmark has also reintroduced many restrictions, including closing primary schools in the lead-up to Christmas.