(CNN) — Vietnam was the only destination added to the US Centers for Disease Control and Prevention’s highest risk Tier 4 category for travel on Monday.

The “very high” risk category still contains more destinations than all the other tiers combined, with nearly 140 places now in Tier 4. At the start of January, there were around 80 destinations listed there.

The CDC places a destination at the risk level “Level 4: Covid-19 very high” when more than 500 cases per 100,000 inhabitants are recorded in the last 28 days.

As a reminder, only one destination was added to level 4 on February 28:

• Vietnam

Vietnam was previously classified as Level 3, or “high” risk.

The number of cases worldwide has been falling since peaking in late January, but experts warn the pandemic is not over.

New Zealand, which has had relatively few Covid cases due to its strict pandemic controls, saw a record number of cases last week. The country remained at Level 3 “high” risk on Monday after falling from Level 2 last week.

CDC: avoid Tier 4 destinations

The CDC advises avoiding travel to Tier 4 countries. The CDC’s thresholds for travel health notices are based primarily on the number of Covid-19 cases in a destination.

Other favorites for tourists stuck at Level 4 for a month or more include Mexico, Canada, France, Peru, Singapore and Spain. The UK has been there since July 2021.

Changes at Level 3

The Level 3 “high” risk category – which applies to destinations that have recorded between 100 and 500 cases per 100,000 population in the past 28 days – saw three additions on Monday. They were:

• Comoros
• Hong Kong
• Sao Tome and Principe

The move was good news for Comoros, an archipelago off the east coast of Africa, and São Tomé and Príncipe, an island nation off the west coast of central Africa, both of which were previously in the level 4.

Levels 2, 1 and unknown

Destinations with the designation “Level 2: Moderate Covid-19” have recorded 50 to 99 cases of Covid-19 per 100,000 inhabitants in the last 28 days.

On Monday there was a lot of good news for Africa. Nine destinations, including Ghana and Uganda, previously listed at Tier 3, have moved to the “moderate” category of Tier 2.

A total of 10 destinations moved 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:

• Ivory Coast
• Ghana
• Lesotho
• Liberia
• Mali
• Montserrat
• Republic of Congo
• Rwanda
• Go
• Uganda

To be at “Level 1: Covid-19 low”, a destination must have registered fewer than 50 new cases per 100,000 inhabitants in the last 28 days.

Nigeria was the only destination moved to Level 1 on Monday. The West African country was previously listed in Tier 2. There are currently only five destinations in the category, including China, which recently hosted the Winter Olympics.

Finally, there are destinations for which the CDC has an “unknown” risk due to lack of information. Usually, but not always, these are small, remote places or places where war or unrest is going on. There were no additions to the category on Monday.

Tanzania, Cambodia, and Macau are some of the most visited places currently listed in the unknown category. The CDC advises against travel to these places precisely because the risks are unknown.

Medical expert weighs in on risk levels

Transmission rates are “a benchmark” for travelers’ personal risk calculations, according to CNN medical analyst Dr. Leana Wen.

“We are entering a phase of the pandemic where people have to make their own decisions based on their medical situation as well as their risk tolerance when it comes to contracting Covid-19,” Wen said in mid -February.

“You have to interpret level 4 to mean that it is a place with a lot of community transmission of Covid-19. So if you go there, you are more likely to contract the coronavirus,” said said Wen, who is an emergency physician and professor of health policy and management at George Washington University’s 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’m vaccinated and boosted, I’m willing to take that risk.’

“So it really has to be a personal decision that people weigh knowing that right now the CDC is categorizing the different tiers based on community transmission rates, and basically just that,” Wen said. “They don’t take into account individual circumstances.”

Top image: Tourists take boat trips on the Thu Bon River on April 24, 2021 in Hoi An, Vietnam. (Linh Pham/Getty Images).