By Kristen Rogers and Marnie Hunter, (CNN) — Switzerland and Puerto Rico are now among the highest-risk destinations for travellers, according to the US Centers for Disease Control and Prevention's regularly updated travel advisories list.

People should avoid travelling to locations designated with the "Level 4: Covid-19 Very High" notice, the CDC recommends. Anyone who must travel should be fully vaccinated first, the agency advises.

Seven destinations moved up on August 30 from the "Level 3: Covid-19 High" list to Level 4:

- Azerbaijan
- Estonia
- Guam
- North Macedonia
- Puerto Rico
- Saint Lucia
- Switzerland

The CDC's evolving list of travel notices ranges from Level 1 ("low") to Level 4 ("very high").

Destinations that fall into the "Covid-19 Very High" Level 4 category have had more than 500 cases per 100,000 residents in the past 28 days, according to CDC criteria. The Level 3 category applies to destinations that have had between 100 and 500 cases per 100,000 residents in the past 28 days.

Switzerland has had 659 laboratory-confirmed Covid-19 cases per 100,000 inhabitants in the past four weeks, according to the country's Federal Office of Public Health. On August 29, nearly a third of Switzerland's intensive care units were occupied by people with coronavirus.

In North Macedonia, slightly less than a quarter of residents were fully vaccinated against Covid-19 as of August 30, while 9 per cent were partially vaccinated. And of Saint Lucia's population of around 185,000 people, it has fully vaccinated 15.1 per cent and partially vaccinated 4.8 per cent.

New 'Level 3' destinations

Ten other destinations moved to the "Level 3: Covid-19 High" category on Monday.

Bermuda, Canada, Germany and Moldova moved up from Level 2. Bahrain, Indonesia, Namibia, Oman, Rwanda and Zimbabwe moved down from Level 4.

CDC guidance for Level 3 destinations urges unvaccinated travellers to avoid nonessential travel to those locations.

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

"Fully vaccinated travellers are less likely to get and spread Covid-19. However, international travel poses additional risks, and even fully vaccinated travellers might be at increased risk for getting and possibly spreading some Covid-19 variants," the agency said.