The U.S. Centers for Disease Control and Prevention on Monday moved four European destinations to its highest-risk category for travel – a reflection of growing concern over rising cases in Europe just as the United States reopens to international travelers from that region.

The CDC gave Hungary, Iceland, the Czech Republic and Guernsey a level 4 warning, which means they are recommending that Americans avoid traveling, even if vaccinated. They join other European destinations on the Level 4 list, including some that were added recently – Luxembourg and the Netherlands for example – and others, such as the United Kingdom, that have been on the list for months.

Countries and territories in this group have an incidence rate of covid-19 of more than 500 new cases per 100,000 people over the past 28 days (or in the case of Guernsey, which has fewer than 100,000 residents, more than 500 cases cumulatively over the past 28 days). As of this week, 81 destinations are listed in the Level 4 category.

The CDC has four levels that start at “low” and escalate to “moderate,” “high” and “very high.” No matter the CDC designation of your destination, the agency says everyone should be fully vaccinated before traveling.

The State Department issues its own travel advisories, which factor in the CDC’s recommendations but also include other threats such as terrorism, civil unrest, crime and natural disasters.

The United States recently reopened its borders to vaccinated international travelers arriving directly from the United Kingdom and most countries in the European Union for the first time in more than 18 months. But the World Health Organization recently declared Europe the latest “epicenter” of the pandemic as cases spike and some countries face a return to lockdowns, raising concerns about the resumption of mass travel.

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The Washington Post’s Hannah Sampson contributed to this report.