U.S. health officials on Monday advised against travel to Canada, citing “very high” levels of the coronavirus in the country.

Canada joins scores of countries that the Centers for Disease Control and Prevention (CDC) has grouped under a Level 4 travel advisory, its most severe, including most of Europe and nations in southern Africa. The agency has also urged unvaccinated people to avoid travel to 54 additional nations, including Mexico – by far the top international destination for U.S. citizens last year, according to federal data. The CDC also issued a Level 4 warning for Curaçao on Monday.

Map: See CDC travel recommendations by destination

New coronavirus cases have risen sharply in Canada in the past few weeks, according to a Johns Hopkins University tracker, as the omicron variant fuels a new global wave of infections.

“If you must travel to Canada, make sure you are fully vaccinated before travel,” the CDC said in its travel alert for Canada, which indicates high prevalence of the virus. “Because of the current situation in Canada, even fully vaccinated travelers may be at risk for getting and spreading COVID-19 variants.”

More on the COVID-19 pandemic

Health officials have emphasized that some vaccinated people will contract the virus, particularly as omicron proves more resistant to existing vaccinations. Research shows that omicron tends to cause milder illness than the delta variant, which became dominant last year. Vaccinations appear to protect against the most serious infections, experts say, especially for those who have gotten a booster shot.

For unvaccinated people who travel, the CDC recommends getting tested for the coronavirus one to three days before a trip. The agency says people should not travel if they are sick; are in isolation after testing positive; are awaiting test results; or are quarantining after a close contact with an infected person.

Canada has generally advised against nonessential international travel.