The U.S. Centers for Disease Control and Prevention (CDC) on Thursday issued a fresh travel warning for Americans going on cruises, telling them to avoid cruise travel, regardless of vaccination status.

The notice comes as the agency has updated its warning on cruise travel from Level 3 to Level 4, the highest such level.

“This reflects increases in cases onboard cruise ships since identification of the Omicron variant,” said the CDC, on its website. 

“Even fully vaccinated travelers may be at risk for getting and spreading COVID-19 variants,” the CDC said. “The virus that causes COVID-19 spreads easily between people in close quarters on board ships, and the chance of getting COVID-19 on cruise ships is very high, even if you are fully vaccinated and have received a COVID-19 vaccine booster dose.”

Along with testing, passengers who are not fully vaccinated should self-quarantine for a full five days after cruise travel, the CDC advised.

“While CDC is exercising its enforcement discretion under CDC’s Mask Order to not require that persons wear a mask under certain circumstances on board foreign-flagged cruise ships subject to the Temporary Extension & Modification of the Framework for Conditional Sailing Order (CSO), including onboard cruise ships choosing to follow the requirements of the CSO on a voluntary basis, individual cruise lines may require travelers (passengers and crew) to wear masks on board the ship,” the agency said.