The U.S. Centers for Disease Control and Prevention (CDC) has lowered its cruise travel warning from Level 4 to Level 3, and specified the warning is now for “travelers who are not fully vaccinated.”

The CDC said it recommends that people who are not fully vaccinated avoid travel on cruise ships, including river cruises, worldwide.

“Since the virus spreads more easily between people in close quarters aboard ships, the chance of getting COVID-19 on cruise ships is high,” the CDC said on its website. “It is especially important that people who are not fully vaccinated with an increased risk of severe illness avoid travel on cruise ships, including river cruises.”