The U.S. Centers for Disease Control and Prevention said people should avoid cruise travel regardless of vaccination status, following a jump in onboard COVID-19 cases, in a major blow to the industry that has been ravaged by the pandemic.

The CDC on Thursday raised its COVID-19 Travel Health Notice level for cruise ships to 4, its highest warning level. There have been several reported outbreaks of coronavirus in the past week on cruise ships, most of which only accept vaccinated passengers

“Even fully vaccinated travelers may be at risk for getting and spreading COVID-19 variants,” the health agency said.

With several cruise ships already on the seas, the CDC said the passengers should get tested three to five days after their trip ends, and self-monitor for COVID-19 symptoms for 14 days.

The CDC has investigated or started an investigation into COVID-19 cases on more than 85 ships already. It had eased its warnings for cruises by a notch from the highest level in June after cases had eased.