By Marnie Hunter and Naomi Thomas, CNN

(CNN) — The US Centers for Disease Control and Prevention on Thursday increased the risk level for cruise ship travel to its highest level and said it should be avoided, regardless of vaccination status.

The agency bumped up the travel risk level for cruise travel from Level 3 to Level 4, indicating the risk for Covid-19 is “very high.”

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The move “reflects increases in cases onboard cruise ships since identification of the Omicron variant,” the CDC website says.

“Since the identification of the Omicron variant, there has been an increase in the number of COVID-19 cases among cruise passengers and crew reported to CDC. Additionally, there has been an increase in the number of cruise ships meeting the COVID-19 case threshold for CDC investigation,” the agency said.

In addition to including cruise travel in its wider list of travel advisories looking at countries worldwide, the agency has created a color status classification for cruise ships.

On December 30, the agency listed 91 ships in its yellow category that indicates that the volume of Covid cases reported has met the CDC’s threshold for investigation. That’s five more ships than were listed as yellow on December 28. On December 30, no ships were listed as red, which indicates that reported cases are at or above the threshold for investigation.

Three ships were listed as orange on December 30, indicating that reported cases are below the CDC’s threshold for investigation. Sixteen ships were listed as green, meaning that the ships had no reported cases of Covid-19.

Green, orange, yellow and red designations indicate a ship’s Covid situation from best to worst. A gray designation is reserved for ships that the CDC has not reviewed for Covid safety.

Decision is ‘perplexing,’ industry association says

Cruise Lines International Association, a trade association, expressed disappointment at the CDC’s elevated risk level.

“The decision by the CDC to raise the travel level for cruise is particularly perplexing considering that cases identified on cruise ships consistently make up a very slim minority of the total population onboard — far fewer than on land — and the majority of those cases are asymptomatic or mild in nature, posing little to no burden on medical resources onboard or onshore,” CLIA said in a statement.

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The association said that “cruise ships offer a highly controlled environment with science-backed measures, known testing and vaccination levels far above other venues or modes of transportation and travel, and significantly lower incidence rates than land.”

At least four ocean cruise ships were turned away from ports of call or were prohibited from letting passengers disembark in the Americas this week because of Covid-19 cases aboard.

Advice if you decide to cruise anyway

While the CDC says to “avoid cruise travel, regardless of vaccination status,” it advises those who do travel on a cruise ship to make sure they are fully vaccinated and boosted, if eligible. People who go on a cruise should also get tested one to three days before their trip, and three to five days after, regardless of vaccination status.

The unvaccinated should also self-quarantine for a five full days after. People on cruise ships should also wear masks in shared places, CDC says.

Most cruise lines are requiring all or the vast majority of passengers and crew to be vaccinated, in compliance with a CDC Conditional Sailing Order that was implemented to allow cruise operations to resume more than a year after the industry came to a standstill at the start of the pandemic.

Many cruise lines also require pre-departure testing, regardless of vaccination status.

Some lines, including Royal Caribbean, require all passengers 2 and older to be masked in indoor public areas, except while actively eating and drinking.

Before coronavirus variants brought a rise in breakthrough cases among the vaccinated, some lines did not require vaccinated passengers to wear masks in areas reserved for vaccinated guests.

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