(CNN) — The central European country of Poland and the central African nation of Niger were among four new additions to the US Centers for Disease Control and Prevention's "very high" risk travel category on Tuesday.
The island nations of Papua New Guinea in the southwestern Pacific and Trinidad and Tobago in the southern Caribbean were also added to Level 4.
Niger moved from low-risk Level 1 to Level 4 on Tuesday, a move the CDC said was because of case counts and not the Omicron variant detected in southern Africa. The other three countries moved up from Level 3, "high" risk.
Typically, the CDC places a destination at Level 4 when more than 500 cases per 100,000 residents are registered in the past 28 days, but the addition of seven southern African nations late last week to the Level 4 category was because of Omicron, not their case numbers. The eighth nation affected by Omicron and the travel ban, Botswana, was already in the Level 4 category.
"The travel restrictions were implemented due to concerns about the Omicron variant, and unknowns surrounding the variant," the CDC said in an email to CNN Travel on Monday afternoon. "There is enough evidence to act to restrict travel and slow the spread of the variant, while still learning more about the degree of threat posed by this variant."
A woman is seen walking in the old city of Agadez, Niger, on September 15.
Michele Cattani/AFP via Getty Images
People should avoid traveling to locations designated with a "Level 4" notice, the CDC recommends. Anyone who must travel should be fully vaccinated first, the agency advises.
Countries added to Level 4 on November 30:
• Papua New Guinea
• Trinidad and Tobago
A question on many of our minds: How should I be thinking about Omicron in terms of travel safety?
There's still a lot scientists don't yet know about the new coronavirus variant, but the best thing anyone can do right now is to get vaccinated, according to Dr. William Schaffner, a professor of infectious diseases at Vanderbilt University Medical Center.
Getting vaccinated is essential for two reasons, he said.
The first is the Delta variant. "That's the virus that's here, right now, in each and every community, spreading," Schaffner said. "But No. 2, and this has to do with Omicron, it is likely that our vaccines will provide at least partial protection. And partial protection is always better than no protection."
The CDC upped its recommendations Monday regarding booster shots for those already vaccinated because of the emergence of the Omicron variant. It is now saying everyone 18 and older should get a booster.
"I strongly encourage the 47 million adults who are not yet vaccinated to get vaccinated as soon as possible and to vaccinate the children and teens in their families as well, because strong immunity will likely prevent serious illness," CDC Director Dr. Rochelle Walensky said.
The other essential safety measure is still wearing a mask when you're out in public.
Schaffner's advice on traveling is largely the same as it was before the discovery of the new variant.
Make sure you're vaccinated, wear a mask, socially distance as much as possible while you're traveling and carefully consider what you're going to do when you get there, which likely puts you at greater risk than the journey itself.
New entries on Level 3
The CDC also made updates to its other categories on Tuesday, which descend in risk to Level 1: Covid-19 low.
The Level 3 category -- which applies to destinations that have had between 100 and 500 cases per 100,000 residents in the past 28 days -- had five updates this week, with a big showing from islands from the Caribbean and Atlantic to the South Pacific.
These five destinations moved to Level 3 on November 30:
• Costa Rica
• French Polynesia
• Saint Lucia
The move was actually good news for all five destinations, which had been at Level 4.
Updates on Level 2
The CDC moved two countries -- Argentina and Djibouti -- down from Level 3 to the less-risky Level 2. Guinea-Bissau in western Africa moved up from Level 1.
These destinations moved to Level 2 on November 30:
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.
Keep in mind the CDC list updates weekly, and the situation in any country can change for better or worse from week to week.
Level 1 and no ratings
In the best news category, four countries moved into the Level 1 "low" risk category on Tuesday.
In "Level 1: Covid-19 Low" destinations, fewer than 50 new cases per 100,000 residents have been logged over the past 28 days.
The four destinations moved into Level 1 on November 30 are:
• British Virgin Islands
• Democratic Republic of the Congo
Finally, there are destinations for which the CDC has an "unknown" risk because of a lack of information. As of November 30, that list included Burundi, Madagascar, Cambodia, Nicaragua and Macau.
In its broader travel guidance, the CDC has recommended avoiding all international travel until you are fully vaccinated.
"Fully vaccinated travelers are less likely to get and spread Covid-19. However, international travel poses additional risks, and even fully vaccinated travelers might be at increased risk for getting and possibly spreading some Covid-19 variants," the agency said.