The U.S. Centers for Disease Control and Prevention just Sid that people should avoid cruise travel, regardless of their vaccination status, as COVID-19 cases continue to surge across the country.

The agency increased its COVID-19 Health Travel Notice for cruise ships to a level 4, the highest warning level, as there's been a recent surge in COVID-19 cases reported on cruise ships across the globe.

"Even fully vaccinated travelers may be at risk for getting and spreading COVID-19 variants," the CDC said on Thursday.

The agency is reportedly investigating COVID-19 cases on more than 85 ships, reports say. In fact, between December 15-29, about 5,000 cases of COVID-19 were reported by cruise ships in the U.S. waters. That's up significantly from just 162 cases in the first two weeks of the month, CNBC reports.

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The news, of course, comes as a blow to the cruise industry, which spent more than a year shuttered before resuming cruising this past June with safety protocols in place across the industry.

Some of the United States' largest cruise lines including Royal Caribbean, Disney, and Carnival did not immediately react to the CDC's announcement on Thursday. Stocks for the major cruise lines tumbled following the news.

However, the week between the Christmas and New Year holidays is typically one of the busiest of the year for the industry.

The CDC says passengers who are currently on ships should get tested for COVID-19 three to five days after their trip ends, and self-monitor for COVID-19 symptoms for 14 days after they return.

An agency representing the major cruise lines, Cruise Lines International Association, said they're disappointed with the CDC's decision.

“We’re obviously disappointed at the CDC’s decision to raise the travel level for cruise today—especially given the overwhelming level of effectiveness of cruise protocols that are resulting in significantly lower level of cases on cruise as compared to land,” Brian Salerno, a representative for the organization, tells CNBC.

It is not clear if any cruise lines will announce any cancelations or revisions to their upcoming itineraries.

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