• The Centers for Disease Control and Prevention increased its travel warning for cruise ships to the highest level.
• The CDC warned that the risk of catching Covid on cruise ships is very high regardless of vaccination status.
• The agency is currently investigating or observing dozens of cruise ships that have had Covid outbreaks.
• Cruise ships operating in U.S. waters reported about 5,000 Covid cases to the CDC between Dec. 15 - 29.

The U.S. Centers for Disease Control and Prevention on Thursday advised people against going on cruises regardless of their vaccination status after a recent surge in positive Covid cases onboard ships as the highly contagious omicron variant sweeps the world.

The CDC increased its travel warning for cruises to the highest level as the agency is investigating or observing dozens of ships that have had Covid outbreaks.

Cruise ships operating in U.S. waters reported about 5,000 Covid cases to the CDC between Dec. 15 - 29, a major spike compared with the first two weeks of the month when 162 cases were reported.

"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 said.

The CDC guidance is a new blow for an industry that was devastated during the first year of the pandemic. The stocks of Royal Caribbean Cruises, Norwegian Cruise Line, and Carnival fell on the news.

The CDC warned that Covid transmits easily between people in close quarters on ships, and the chance of catching the virus on a cruise is very high even for people who are vaccinated and have received a booster dose.

The CDC advised people who decide to go on a cruise to get vaccinated before their trip and receive a booster dose if eligible. Facemasks should also be worn in shared spaces, and passengers who are not fully vaccinated should self-quarantine for five days after travel, according to the agency.

The CDC also said people who go on a cruise should get tested one to three days before departing, and three to five days after their trip, regardless of vaccination status or symptoms. However, many Americans shopping for at-home tests have found shelves in many pharmacies empty amid a nationwide shortage, and lines at clinics are sometimes hourslong.

Brian Salerno with the Cruise Lines International Association said the trade group was frustrated by 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," said Salerno, senior vice president for maritime policy.

Anne Madison, a spokesperson for the industry group, said the Covid cases identified on cruise ships make up a slim majority of the total population onboard.

"The majority of those cases are asymptomatic or mild in nature, posing little to no burden on medical resources onboard or onshore," Madison said.

Covid cases in the U.S. have surged to a pandemic high. The U.S. reported a seven-day average of more than 300,000 daily new Covid cases as of Wednesday, an increase of 82% from last week, according to data compiled by Johns Hopkins University. CDC Director Dr. Rochelle Walensky said Wednesday that omicron is driving the rapid increase in Covid cases.

White House chief medical advisor Dr. Anthony Fauci on Wednesday said data out of South Africa and the United Kingdom, countries that were hit by omicron earlier, indicate that the variant is less severe than delta. However, Fauci urged against complacency, warning that omicron could still cause a spike in hospitalizations that burdens health-care systems simply by transmitting much faster.

About 82,000 Americans are hospitalized with Covid as of Thursday, according to a seven-day average of data from the Department of Health and Human Services, up 17% over the past week.

Correction: Norwegian Cruise Line and Carnival stocks were down more than 2% on the news. Royal Caribbean fell 1.93%. A previous version misstated Royal Caribbean's stock move.