CDC data suggests the majority of 2024 gastrointestinal outbreaks at sea were associated with the highly contagious norovirus.

In December alone, five cruise ships were stricken with the vomiting bug, with hundreds of passengers reporting symptoms on Cunard Line, Holland America and Princess Cruises’ vessels.

When an outbreak appears in CDC’s data, it means three per cent or more of the ships’ passengers and crew presented gastrointestinal symptoms to medical staff.

Norovirus has long been associated with cruise ships because the virus spreads in environments where people are in close proximity. Infectious disease expert Dr William Schaffner, a professor at Vanderbilt University Medical Centre, notes norovirus is “known obviously as the cruise ship virus.”

“This is a virus that can infect you with very few viral particles. In other words, the infectious dose is very small,” Schaffner told CNN Travel.

“It can persist for days or even a week on environmental surfaces, which means that if you put your fingers on a contaminated surface, you can pick up a few viral particles, touch your mouth and then initiate an infection.”

Norovirus tends to peak during winter months, and “this seasonal occurrence of norovirus varies in intensity,” explains Schaffner.

This season’s occurrence is “more severe” than average — this is true both on land and at sea, according to Schaffner, who adds this may explain the CDC data from December 2024.

“Incidents of illness onboard cruise ships are extremely rare,” industry organisation Cruise Lines International Association said in a statement.

While norovirus dominates the 2024 CDC data, a September 2024 gastrointestinal outbreak on Royal Caribbean’s Radiance of the Seas was attributed to salmonella food poisoning. Meanwhile, Silversea Cruises’ outbreak on Silver Nova in the spring was the result of E coli and the cause of gastrointestinal illness on Royal Caribbean’s Allure of the Seas’ May voyage remains unknown.

The CDC reported a total of 16 gastrointestinal cruise outbreaks across 2024, the highest in over a decade. In 2023, there were a total of 14 reported outbreaks.

But speaking to CNN Travel, a CDC spokesperson says that “while 2023 and 2024 both had higher numbers of cruise ship outbreaks than in years prior to the pandemic, we do not yet know if this represents a new trend.”

There were fewer reports in the 2020-2022 period, during which time the cruise industry shut down — and then slowly restarted — in the wake of the COVID-19 pandemic. Back in 2019, there were 10 reported outbreaks, according to CDC data.

“Cruise ship travel during the COVID-19 pandemic was limited, and prior to that, we saw that the rates of gastrointestinal illness on cruise ships decreased (during 2006-2019),” says the CDC spokesperson.

CDC data only covers ships under the agency’s jurisdiction — meaning the ship’s voyage must include a US port, a foreign itinerary and be carrying more than 13 passengers. While this criteria counts for a lot of the world’s cruise ship fleet, it doesn’t cover every vessel traversing the world’s oceans.

The CDC spokesperson recommends cruise passengers who present stomach bug symptoms can help limit the spread of infection by “promptly reporting their illness if they are sick and following medical staff recommendations.”

Passengers are also advised to practice “washing their hands often, especially after using the toilet and before eating or drinking.”

Cruise ships also pride themselves on taking steps to control outbreaks — from quarantining passengers and crew with symptoms, to increasing cleaning and disinfection of impacted vessels.

Infectious disease expert Schaffner says that due to the supremely infectious nature of norovirus, “it is likely to go beyond all those barriers and interventions that have been put in place” by cruise companies.

Schaffner adds that experts can’t predict if the recent wave of norovirus in the US will continue into 2025 and beyond, whether at sea or on land.

“Whether this surge that we’re seeing now will extend into the next season, or the next season will be, once again, a low season I would be loath to predict,” he says.

“I think we’ll just have to wait and see on an individual basis.”

In the meantime, Schaffner’s advice to prospective cruise passengers is “number one, if you’re feeling at all ill, stay home, rebook for a later cruise”.

“Number two, pay meticulous attention to all the hygienic instructions that you are given on the cruise ship, and pay particular attention to hand hygiene,” he said.

“And in this circumstance, soap and water is actually better than using the sanitary hand wipes or lotions that we use, because norovirus is not very affected by the alcohol that’s in the hand wipes and the lotions.”