(CNN) – Puerto Rico and Switzerland are now among the most dangerous destinations for travelers due to COVID-19, according to the list of travel warnings from the US Centers for Disease Control and Prevention (CDC, for short in English), which is updated periodically.

The Centers for Disease Control and Prevention (CDC) recommends avoiding travel to places designated by the “Level 4: Covid-19 very high” advisory. The agency warns that anyone who has to travel should be fully vaccinated first.

On August 30, seven destinations were upgraded from the “Level 3: Hai COVID-19” list to Level 4:

• Azerbaijan
• Estonia
• Guam
• North Macedonia
• Puerto Rico
• Lucia Street
• Switzerland

The Centers for Disease Control and Prevention’s (CDC) evolving travel advisory list ranges from Tier 1 (“low”) to Tier 4 (“very high”).

Destinations that fall under 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 the CDC’s criteria. The Tier 3 category applies to destinations that have between 100 and 500 cases per 100,000 residents in the last 28 days.

Switzerland has recorded 659 laboratory-confirmed cases of COVID-19 per 100,000 residents in the past four weeks, according to the country’s Federal Office of Public Health. On August 29, around a third of Switzerland’s intensive care units were staffed with coronavirus patients. In North Macedonia, less than a quarter of the population had been fully vaccinated against COVID-19 as of August 30, while 9% had been partially vaccinated. Of St Lucia’s population of about 185,000 people, 15.1% have been fully vaccinated and 4.8% have been partially vaccinated.

New ‘tier 3’ destinations

Ten other destinations were upgraded to the “Level 3: Covid-19 high” category on Monday.

Bermuda, Canada, Germany and Moldova rose from Level 2. Bahrain, Indonesia, Namibia, Oman, Rwanda and Zimbabwe all fell from Level 4.

The CDC’s Guide to Tier 3 Destinations urges unvaccinated travelers to avoid non-essential travel to those locations.

In its broader travel guide, the CDC recommends avoiding international travel until fully vaccinated.

“Fully vaccinated travelers are less likely to contract and spread COVID-19. However, international travel presents additional risks, and even fully vaccinated travelers may be at greater risk of infection and possibly spreading some types of COVID-19,” the agency said.

You can see the CDC risk level for any destination in Travel Agency Recommendation Page.