Originally Published: 06 JUN 22 16:49 ET

By Forrest Brown and Marnie Hunter, CNN

(CNN) -- The US Centers for Disease Control and Prevention has added four places to its list of "high" risk destinations for Covid-19, including a small and charming Caribbean nation.

St. Kitts and Nevis, part of the Leeward Islands east of Puerto Rico, was placed on the Level 3 category on Monday. The destination is known for its gorgeous, lush scenery and cultural experiences.

In April, the CDC overhauled its ratings system for assessing Covid-19 risk for travelers.

The Level 3 "high" risk category is now the top rung in terms of risk level. Level 2 is considered "moderate" risk, and Level 1 is "low" risk.

Level 4, previously the highest risk category, is now reserved only for special circumstances, such as extremely high case counts, emergence of a new variant of concern or health care infrastructure collapse. Under the new system, no destinations have been placed at Level 4 so far.

The "Level 3: Covid-19 High" category now applies to countries that have had more than 100 cases per 100,000 residents in the past 28 days.

Here's the full list of new arrivals to Level 3 on Monday:

• Guyana

• Mongolia

• Namibia

• St. Kitts and Nevis

All had previously been at Level 2.

There were about 115 destinations at Level 3 on June 6. Level 3 locations now account for almost half of the roughly 235 places monitored by the CDC.

Level 3

Many other popular travel destinations are also at Level 3.

Much of Europe has been stubbornly lodged there for months as the summer travel season starts. As of June 6, the following popular European destinations were among those remaining at Level 3:

• France

• Germany

• Greece

• Ireland

• Italy

• The Netherlands

• Portugal

• Spain

• United Kingdom

It's not just European favorites that find themselves at Level 3. Numerous notable travel destinations around the world are among those in the high risk category, including the following:

• Brazil

• Canada

• Costa Rica

• Malaysia

• South Korea

• Thailand

The CDC advises that you get up-to-date with your Covid-19 vaccines before traveling to a Level 3 destination. Being "up-to-date" means you have had not only the full initial vaccinations but any boosters for which you're eligible.

Level 2

Destinations carrying the "Level 2: Covid-19 Moderate" designation reported 50 to 100 Covid-19 cases per 100,000 residents in the past 28 days. Just two places were moved to this level on Monday:

• Guatemala

• Zimbabwe

Home to beautiful Lake Atitlán and Mayan ruins, Guatemala was at Level 3 last week. The move was bad news for safari destination Zimbabwe in southern Africa. It had been at Level 1.

There were about 15 destinations listed in Level 2 on June 6.

You can view the CDC's risk levels for any global destination on the agency's travel recommendations page.

In its broader travel guidance, the CDC has recommended avoiding all international travel until you are fully vaccinated.

If you're concerned about a travel-specific health situation not related to Covid-19, check here.

Level 1

To be in "Level 1: Covid-19 Low," a destination must have had 49 or fewer new cases per 100,000 residents over the past 28 days. Just two destinations were added to the category on June 6:

• Iran

• Lebanon

Both Middle Eastern nations were at Level 2 last week.

Unknown

Finally, there are destinations the CDC has deemed to be of "unknown" risk because of a lack of information. Usually, but not always, these are small, remote places or places with ongoing warfare or unrest.

Libya was the only place added to this category on June 6.

The CDC advises against travel to these places precisely because the risks are unknown. Destinations in this category include Cambodia, the Canary Islands, Macau and Tanzania.

A medical expert weighs in on risk levels

Transmission rates are just "one guidepost" for travelers' personal risk calculations, according to CNN Medical Analyst Dr. Leana Wen.

We've moved into "a phase in the pandemic where people need to make their own decisions based on their medical circumstances as well as their risk tolerance when it comes to contracting Covid-19," said Wen, who is an emergency physician and professor of health policy and management at the George Washington University Milken Institute School of Public Health.

There are other factors to weigh in addition to transmission rates, according to Wen.

"Another is what precautions are required and followed in the place that you're going and then the third is what are you planning to do once you're there," she said.

"Are you planning to visit a lot of attractions and go to indoor bars? That's very different from you're going somewhere where you're planning to lie on the beach all day and not interact with anyone else. That's very different. Those are very different levels of risk."

Vaccination is the most significant safety factor for travel, since unvaccinated travelers are more likely to become ill and transmit Covid-19 to others, Wen said.

And it's also important to consider what you would do if you end up testing positive away from home. Where will you stay and how easy will it be to get a test to return home?