Every Monday, the Centers for Disease Control and Prevention delivers news for travelers — namely, which destinations they should avoid because of covid-19.

This week, Switzerland, Puerto Rico and others were put on that list, and the week before, Bahamas and St. Maarten were added. Earlier this month, countries including France, Iceland and Greece found themselves in that “Level 4” category, which signifies that covid-19 is very high and people should stay away.

“Travel increases your chance of getting and spreading COVID-19,” CDC spokeswoman Caitlin Shockey said in an email. “You may feel well and not have any symptoms, but you can still spread COVID-19 to others. Staying home is the best way to protect yourself and others from COVID-19.”

At the same time, 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 U.S. State Department and CDC’s travel advisories are accurate and up to date,” Abinash Virk, an infectious-disease specialist and former head of the travel clinic at the Mayo Clinic in Minnesota, said in an email. “They are reliable resources and extremely helpful to determine if travel to a certain location will be safe from COVID-19 perspective or not.”

But to navigate all the advice, experts say, travelers must read the details — and understand what the warnings describe.

What are the CDC’s levels?

The CDC has four levels that start at “low” and escalate to “moderate,” “high” and “very high.” A country or territory that does not provide data is categorized as “unknown.” No matter the CDC designation of your destination, the agency says everyone should be fully vaccinated before traveling.

Beyond that, the warnings start to escalate at Level 2, or “moderate” levels of covid-19. When a destination has a Level 2 status, the CDC advises that unvaccinated travelers who are at high risk for severe illness should avoid nonessential travel.

One step up, at Level 3, the CDC says all unvaccinated travelers should avoid nonessential travel. For destinations marked as Level 4 or unknown, the agency says Americans should avoid traveling there altogether.

How are the levels decided?

The CDC’s destination-specific recommendations are based on the number of new cases reported per 100,000 people over the past 28 days.

That metric is more useful than a case count alone because it gives a better idea of someone’s risk of getting infected, said Lise Barnard, a health-intelligence analyst at risk management firm Crisis24.

“This can be very informative when deciding when to travel, and travel should be reconsidered in countries with a high or very high risk of transmission, or to countries that have a higher risk profile [than] the individual’s country of origin,” Barnard wrote in an email.

People who are especially vulnerable to severe illness should also weigh their own risks, she said, even when traveling to a place that is rated at a lower level.

How are the State Department’s levels different?

Because the State Department’s travel advisories are based in part on CDC assessments, it might put a destination on the same level, also on a scale of 1-4. But it might not. Level 1 means travelers should exercise normal precautions, which graduates to exercising increased caution for Level 2 and reconsidering travel for Level 3. Level 4 means “do not travel.”

“In addition to CDC’s advice, the department also takes into account logistical factors, including in-country testing availability and current restrictions on entry for U.S. citizens, when determining each country’s travel advisory level,” the department said in a statement. “So the department’s travel advisory level may not always match the CDC’s [travel health notice] level.”

Still, many countries are at a Level 3 or 4 according to both the CDC and the State Department, including destinations that are popular with American travelers. The State Department has put France and the Bahamas, for instance, in the same Level 4 (“Do Not Travel”) category as places that are not travel destinations, such as Iraq and Afghanistan.

Each State Department travel advisory specifies why a country is placed in a given level and what health, safety or security concerns to keep in mind.

“In our travel information, we warn people not to visit certain high-risk countries and areas both because of local conditions and because we are limited in our ability to provide consular services in those places,” the department said.

What are other sources to check?

Virk, of Mayo Clinic, recommended checking country-specific information about covid-19 on the World Health Organization’s website. She also said travelers should talk to a travel medicine expert to help them assess their risks.

“This is particularly important for individuals who have comorbidities that potentially increase the risk of severe COVID-19,” she said in an email.

The State Department publishes broader country information pages in addition to its travel advisories.

Those pages also include links to each embassy’s covid-19 page, which include entry and exit requirements, testing availability, vaccine information, curfews and other pandemic-related information.

What are the risks of leaving the U.S.?

Given the state of the pandemic in the United States, where community transmission is high in most counties, travelers might wonder whether traveling abroad is really an increased risk. But experts say there are many reasons to think carefully about going abroad.

“Even if the situation in the U.S. is less than desirable, traveling always poses a risk,” Barnard said. “New variants may emerge and can be transmitted when moving between locations, especially in areas with high transmission, as was observed with the emergence of the delta variant in India.”

And, the CDC says, travel increases the chance of contracting and spreading the virus.

“Travelers need to be aware that they can spread disease at their destination among people who may not have the same access to vaccinations and quality medical care,” the CDC’s Shockey said.