(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 (CDC) updated its international travel guide on Monday to provide specific advice for vaccinated and unvaccinated travelers.

The update includes moving 33 countries, including Iceland, Israel and Singapore, to the lowest risk category.

The revised CDC Covid-19 Travel Health Advisory guidelines seek to “better differentiate countries with severe outbreak situations” from those in which Covid-19 is “sustained, but controlled.”

CDC 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 depending on the severity of the pandemic in each country.

The CDC recommends avoiding travel to countries at level 4, the highest threat level, where there are more than 500 cases per 100,000 residents in the past 28 days. Tier 4 countries include nations such as Brazil, India, and Iraq.

For countries at level 3, such as Mexico, Russia, and Iran, the CDC recommends against non-essential travel for those who are not vaccinated. These countries are currently reporting 100 to 500 cases per 100,000 inhabitants.

At level 2, the agency recommends that unvaccinated travelers who are at severe risk of contracting a serious illness from COVID-19 should avoid visiting. These nations, such as Finland, Cambodia and Kenya, are currently reporting between 50 and 99 cases per 100,000.

Lastly, Tier 1 countries like Australia and New Zealand are considered the lowest risk destinations, reporting fewer than 50 COVID-19 cases in the past 28 days.

The CDC still recommends getting vaccinated before traveling to a low-risk location.

See the CDC Travel Health Advisory here, and the CDC interactive travel map here.