(CNN) — Vietnam was the only destination added to the US Centers for Disease Control and Prevention's highest-risk Level 4 category for travel on Monday.
The "very high" risk category still contains more destinations than all the other levels put together, with nearly 140 places now at Level 4. In early January, there were around 80 destinations listed there.
The CDC places a destination at "Level 4: Covid-19 Very High" risk when more than 500 cases per 100,000 residents are registered in the past 28 days.
To recap, a single destination was added to Level 4 on February 28:
Vietnam was previously listed at Level 3, or "high" risk.
Global case numbers have been declining since peaking in late January, but experts caution that the pandemic is not over.
New Zealand, which has had relatively few Covid cases due to its strict pandemic controls, has recorded record numbers of cases in the past week. The country remained at "high" risk Level 3 on Monday after moving up from Level 2 last week.
Vietnam was moved to the CDC's highest-risk category for travel on Monday. Here, a striking landscape of terraced rice fields is pictured in northern Yen Bai province.
Nhac Nguyen/AFP via Getty Images
CDC: Avoid Level 4 destinations
The CDC advises avoiding travel 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 does not include the United States in its list of advisories, but it was color-coded at Level 4 on February 28 on the agency's map of travel risk levels.
Other tourist favorites stalled on Level 4 for a month or more include Mexico, Canada, France, Peru, Singapore and Spain. The United Kingdom has been there since July 2021.
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 three additions on Monday. They were:
• Hong Kong
• São Tomé and Príncipe
The move was good news for Comoros, an archipelago off Africa's east coast, and São Tomé and Príncipe, an island nation off the west coast of Central Africa, which had both previously been at Level 4.
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, there was a lot of good news for Africa. Nine destinations, including Ghana and Uganda, previously listed at Level 3 moved down to the "moderate" Level 2 category.
In total, 10 destinations moved down to Level 2 on Monday. The tenth, Montserrat, is a British Overseas Territory in the Caribbean.
Here are the 10 destinations moved to Level 2 on February 28:
• Côte d'Ivoire
• Republic of the Congo
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.
Nigeria was the sole destination moved to Level 1 on Monday. The country on the western coast of Africa was previously listed at Level 2. There are currently only five destinations in the category, including China, which recently hosted 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 to the category on Monday.
Tanzania, Cambodia and Macau 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.
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 in mid-February.
"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."