(CNN) As more people get vaccinated and the spread of Covid-19 becomes more controlled, public health officials in the United States are issuing new travel advice for more than 120 countries.
The US Centers for Disease Control and Prevention updated its international travel guidance on Monday to give more country-specific advice for both vaccinated and unvaccinated travelers.
The update includes moving 33 countries, including Iceland, Israel and Singapore, into the lowest risk category.
The CDC's Covid-19 revised Travel Health Notice guidelines seek to "better differentiate countries with severe outbreak situations" from countries in which Covid-19 is "sustained, but controlled."
The CDC's threat levels are determined by the number of Covid-19 cases in a given country. At each level, the CDC recommends getting vaccinated, but its guidance varies by how severe the pandemic is in each country.
The lowest-risk, level 1 countries have low incidence rates of Covid-19 (fewer than 50 new cases over the past 28 days per 100,000 people) or low case counts (fewer than 50 cases over the past 28 days in countries with populations of 100,000 or fewer).
The CDC also looks at whether cases have decreased, increased or remained stable over 28 days as a primary criteria in determining levels. Testing data is a secondary factor in determining risk levels.
The Caribbean is gaining steam as a low-risk region.
Anguilla, Antigua and Barbuda, Dominica, Saint Barthélemy and Turks and Caicos are all new additions to the lowest-risk CDC category. The Central American country of Belize, located along the Caribbean Sea, was also added.
Those destinations join Grenada, Montserrat, Saint Kitts and Nevis, Sint Eustatius and the Cayman Islands, which were already in the lowest risk category.
Testing and quarantine requirements are still in place for some of these destinations, according to CNN's list of international destinations open to Americans. New consideration is being given to vaccinated travelers, but being fully vaccinated does not necessarily mean exemption from other safety measures.
Europe, which is starting to open up to international travelers this summer, is very sparsely represented on the level 1 list. The tiny island nation of Malta, the self-governing Isle of Man in the Irish Sea, Albania, Iceland and Greenland are in the lowest-risk category.
Many of Europe's top tourist destinations -- Spain, Italy, France, Germany and Greece -- are in the CDC's level 3, higher risk category. Belgium, Croatia and the Netherlands are in the highest risk, level 4 category.
French Polynesia, Morocco and Singapore are among popular tourist destinations in other regions that have been designated a low-risk level 1.
The CDC recommends avoiding travel to countries at level 4, the highest threat level, which have more than 500 cases per 100,000 residents in the last 28 days. Other level 4 countries include nations such as Brazil, India and Iraq.
For countries at level 3, which also includes Mexico, Russia and Iran, the CDC recommends against nonessential travel for that those who are unvaccinated. These countries are currently reporting 100 to 500 cases per 100,000 residents.
At level 2, the agency recommends that unvaccinated travelers who are at severe risk for severe illness from Covid-19 should avoid visiting. These nations, like Finland, Cambodia and Kenya, are currently reporting 50 to 99 cases per 100,000.
While level 1 countries are considered the lowest risk destinations and have reported less than 50 Covid-19 cases per 100,000 people in the last 28 days, the CDC still recommends getting vaccinated before traveling to these locations.
See the CDC's Travel Health Notice here, and the CDC's interactive travel map here.