The Carnival Cruise Line’s Carnival Sunrise ship is seen in the port of Miami on Dec. 23, 2020, amid the coronavirus pandemic.

Daniel Slim/AFP via Getty Images

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Daniel Slim/AFP via Getty Images

The Carnival Cruise Line’s Carnival Sunrise ship is seen in the port of Miami on Dec. 23, 2020, amid the coronavirus pandemic.

Daniel Slim/AFP via Getty Images

The Centers for Disease Control and Prevention issued a new advisory Thursday that travelers should avoid traveling by cruise ship, regardless of vaccination status, after a recent surge in positive COVID-19 cases onboard ships.

The agency increased its travel warning for cruises to Level 4 — the highest level — following investigations of dozens of ships that have had outbreaks of the virus.

“Even fully vaccinated travelers may be at risk for getting and spreading COVID-19 variants,” the CDC said on its website.

Following 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 the agency, the CDC said.

More cruise ships have reached “level yellow” — the level where the CDC investigates a ship’s COVID outbreak.

“It is especially important that travelers who are at an increased risk of severe illness from COVID-19 avoid travel on cruise ships, including river cruises, worldwide, regardless of vaccination status,” the agency added.

There are 91 cruise ships currently under investigation or observation, according to the CDC website. However, the agency has not specified how many COVID-19 cases have been reported, according to The Associated Press.

An additional three ships are being monitored.

The CDC advised that those who choose to travel on a cruise should get vaccinated against COVID-19 before their trip and, if eligible, receive a booster dose.

In addition, masks should also be worn in shared spaces and passengers who are not fully vaccinated against COVID should self-quarantine for five days after travel, in addition to getting tested three to five days after their return, the CDC said.

In an email to NPR, a spokesperson for the Cruise Lines International Association said the trade group was “disappointed” by the CDC’s latest advisory against cruise ship travel.

“While we are disappointed and disagree with the decision to single out the cruise industry—an industry that continues to go above and beyond compared to other sectors—CLIA and our ocean-going cruise line members remain committed to working collaboratively with the CDC in the interest of public health and safety,” the trade group said.

Most cruise lines require adult passengers to show proof of vaccination against COVID-19, according to AP.

For now, CDC guidance allows cruise ships to relax measures — such as mask usage — if at least 95% of passengers and 95% of the cruise ship crew are fully vaccinated against the coronavirus.