On Thursday, the Centers for Disease Control and Prevention recommend that travelers avoid cruise ships, regardless of vaccination status.

The CDC updated cruise ships' travel health notice levels from Level 3 to its highest level, Level 4.

The new CDC guidance comes as the highly-contagious omicron variant spreads across the country, leading some hospital systems to become overtaxed with patients.

"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," the CDC's update said.

The new guidance came after the agency said Wednesday that 88 vessels were either under investigation or observation for potential COVID-19 outbreaks. The CDC did not specify how many cases have been reported.

None of the ships appeared to have so many cases they would overwhelm medical resources on board and require a return to port, but some have been denied entry at some foreign ports.

Connecticut Democratic Sen. Richard Blumenthal called for the CDC and cruise lines to again halt cruise travel six months after the industry mounted its comeback.