(CNN) -- As more people get vaccinated and the spread of COVID-19 becomes more controlled, public health officials 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 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 for unvaccinated people varies by how severe the pandemic is in each country.
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. Level 4 countries include nations such as Brazil, India and Iraq.
For countries at level 3, like 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 50-99 cases per 100,000.
Finally, level 1 countries like Australia and New Zealand are considered the lowest risk destinations, and have reported less than 50 COVID-19 cases in the last 28 days. The CDC still recommends getting vaccinated before traveling to a low-risk location.