People over age 60 or with underlying health conditions should not travel internationally because of Omicron, the World Health Organization warned Tuesday.

The new guidance was issued as part of the WHO’s updated travel advice, which cited “preliminary evidence” that the new COVID-19 variant posed “an increased risk of reinfection.”

The strain, first reported last week by scientists in South Africa, contains a number of mutations not found in the widespread Delta variant of the virus.

Scientists fear that those mutations could lead to an increase in breakthrough cases among those immunized against the disease or who have recovered from earlier variants.

Omicron has been reported in at least 17 countries and territories as of Tuesday.