By Marnie Hunter and Forrest Brown, CNN
(CNN) -- The US Centers for Disease Control and Prevention designated a half dozen new places "very high" risk for travel on Monday, including South Korea and French Polynesia.
The six new additions to the "very high" risk Level 4 category are a far-flung bunch: Azerbaijan on the Caspian Sea; Belarus, located in a presently very tense spot bordering Russia and Ukraine; Comoros, an archipelago off Africa's east coast; French Polynesia in the South Pacific; Saint Pierre and Miquelon, a French archipelago south of Canada's Newfoundland; and South Korea.
Level 4, the CDC's highest risk level, has now swelled to almost 140 places, illustrating the rapid surge of the Omicron variant around the world. In early January, there were around 80 destinations listed there.
Level 4 now has more destinations than all the other CDC categories combined and accounts for more than half of all the destinations listed by the CDC.
Destinations added to Level 4 on February 14:
• French Polynesia
• Saint Pierre and Miquelon
• South Korea
Last week, all six destinations were at Level 3, or "high" risk for Covid-19.
The CDC advises travelers to avoid visiting to Level 4 countries. CDC thresholds for travel health notices are based primarily on the number of Covid-19 cases in a destination.
The CDC places a destination at Level 4 when more than 500 cases per 100,000 residents are registered in the past 28 days.
A medical expert weighs in on risk levels
Transmission rates are "one guidepost" for travelers' personal risk calculations, according to CNN Medical Analyst Dr. Leana Wen.
"We are entering 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," Wen said on Monday.
"You should interpret Level 4 to mean this is a place with a lot of community transmission of Covid-19. So if you go, there is a higher chance that you could contract the coronavirus," 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.
Some people will decide the risk is too high for them, Wen said. "Other people will say, 'Because I am vaccinated and boosted, I am willing to take on that risk.'
"So this really needs to be a personal decision that people weigh understanding that right now the CDC is classifying the different levels based on community transmission rates, and basically only that," Wen said. "They're not taking into account individual circumstances."
Other significant places at Level 4
The CDC does not include the United States in its list of advisories, but it was color-coded at Level 4 on February 14 on the agency's map of travel risk levels.
Last week, Japan was among the big tourist destinations added to Level 4.
Other tourist favorites parked on Level 4 even longer include Mexico, Canada, France, Peru, Singapore and Spain. The United Kingdom has been there since July 2021.
You can view the CDC's risk levels for any global destination on its travel recommendations page.
In its broader travel guidance, the CDC has recommended avoiding all international travel until you are fully vaccinated.
Changes at Level 3
The Level 3 "high" risk category -- which applies to destinations that have had between 100 and 500 cases per 100,000 residents in the past 28 days -- saw four additions on Monday. They were:
• El Salvador
Eswatini and Mauritius dropped down one rung from Level 4. El Salvador's risk level increased from its previous position at Level 2, and Indonesia jumped up two risk levels from Level 1.
Levels 2, 1 and unknown
Destinations carrying the "Level 2: Covid-19 Moderate" designation have seen 50 to 99 Covid-19 cases per 100,000 residents in the past 28 days. On Monday, the CDC updated its guidance on Pakistan, which was moved to Level 2 last week. There were no further changes to Level 2.
Currently, there are just four destinations at Level 2, including New Zealand, which has some of the world's tightest travel restrictions.
To be in "Level 1: Covid-19 Low," a destination must have fewer than 50 new cases per 100,000 residents over the past 28 days.
No destinations were moved to Level 1 on Monday. There are currently only six destinations in the category. That includes China, which is hosting the Winter Olympic Games.
Finally, there are destinations for which the CDC has an "unknown" risk because of a lack of information. Usually, but not always, these are small, remote places or places with ongoing warfare or unrest. There were no additions this week.
Tanzania, Cambodia and the Canary Islands are among the more-visited locations currently listed in the unknown category. The CDC advises against travel to these places precisely because the risks are unknown.
The CDC includes cruise ships on its destinations list. Cruise travel has been at Level 4 since December 30. The CDC guidance is to avoid cruise travel at this time.
"It is especially important that travelers who are at an increased risk of severe illness from Covid-19 avoid travel on cruise ships, including river cruises," the CDC says in its travel notice.
On February 9, the CDC updated its travel health notice for cruise travel adding criteria for how it determines the risk level for cruises. Cruise travel remained at Level 4 in the February 9 update.
For cruise travel, the CDC's primary criteria in evaluating the risk level is the number of new Covid-19 cases among crew and the case trajectory among crew over the past 14 days.
Level 4 means more than 2,000 cases detected among crew on cruise ships over the past 14 days. Level 3 is 1,000 to 2,000 news cases. Level 2 is 500-999 new cases and Level 1 is fewer than 500 new crew cases.
Meanwhile, the CDC's Covid-19 risk mitigation guidance has become optional for many cruise ships.
The CDC's extended conditional sailing order expired In January, and the agency has transitioned to a voluntary program for foreign-flagged cruise ships operating in US waters.
More considerations for travel
Transmission rates are important to consider when making travel decisions, but there are other factors to weigh as well, according to Dr. Wen.
"The transmission rates are one guidepost," Wen said. "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.
"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.
"People who are unvaccinated remain at high risk and really should not be traveling at this point," she said.
People should be wearing a high-quality mask -- N95, KN95 or KF94 -- anytime they're in crowded indoor settings with people of unknown vaccination status, she 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?
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