The CDC and State Department have issued travel alerts for Germany and Denmark due to rising rates of COVID-19.
The two E.U. nations were placed by the Centers for Disease Control and Prevention's Level 4 category, meaning there is a "very high" chance of contracting the virus if traveling in either place.
"Avoid travel to these destinations," the CDC website reads. "If you must travel to these destinations, make sure you are fully vaccinated before travel."
Other E.U. countries on the CDC's level 4 list are Austria, Belgium, Ireland, Romania, Slovakia, Switzerland, the Czech Republic and the United Kingdom.
Ahead of the holiday, Germany has decided to limit large parts of public life in places in which hospitals are full of COVID patients, though the country has reached a nearly 70% vaccination rate.
Austria, Germany's neighbor, returned Monday to full lockdown mode after announcing renewed restrictions that target unvaccinated people. Thousands of officers have been placed in high-traffic areas to conduct spot checks of those who are out and about.
German acting Health Minister Jens Spahn warned last week that the may soon follow suit. Danish officials, meanwhile, have suggested a bill that would mandate a digital COVID-19 vaccine passport system for employees, though several elected officials have cautioned against such a system arguing that it would effectively create two tiers of society.