All of the 92 cruise ships have reported enough cases to warrant an investigation. For cruise ships with passengers, this means 0.10% or more of passengers and one or more crew members have caught the virus or that the ship over a period of seven days did not report lab results to the CDC for a day or more.

The agency now advises even fully-vaccinated people to avoid cruises. Last week, it updated its travel health notice for cruise ships to level 4, the highest level. It had previously cautioned against cruise travel for the immunocompromised and those who had not been fully vaccinated.

“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 agency said on its website. 

Cases on cruise ships have been steadily increasing. In a statement from late December, the CDC reported that 5,013 COVID cases had been reported on cruise ships operating in U.S. waters between Dec. 15 and 29, up from just 162 cases during the first two weeks of December.

Norwegian Cruise Line, one of the largest players in the industry, on Wednesday canceled a nine-day cruise to the Caribbean hours before the ship was set to leave because of “COVID related circumstances.” The company also recalled the Norwegian Pearl to port on Wednesday after one day of what was supposed to be a nine-day journey. 

As of Wednesday the company has canceled voyages on eight ships, because of “ongoing travel restrictions.” Another large cruise company, Royal Caribbean, announced Friday it was also canceling some sailings and pushing back one ship’s return to sea.

Some Caribbean countries are tightening their rules for arriving cruise passengers based on the increased COVID cases. Passengers arriving in Barbados, for example, must be fully vaccinated against COVID and have tested negative three days before entering the country to be able to freely visit. Those who are vaccinated but don’t have the test can only visit as part of limited “bubble tours” either operated by the cruise line or third-party operators in the country.

Three of the largest companies in the cruise ship industry, Norwegian, Royal Caribbean, and Carnival, all lost money in 2021 and are expected to suffer losses again this year. All the companies’ stock prices dropped last week after the CDC changed its guidance, although they have slightly recovered this week.

Cases of the Omicron variant have exploded in the U.S. The country was reporting a daily average of 600,000 cases daily as of Friday, according to the New York Times.