Despite stringent measures supposed to keep ocean cruises Covid-free, operator Royal Caribbean says at least 48 people on board one of its ships that docked in Miami at the weekend have tested positive for the virus.

The Symphony of the Seas, the world’s biggest cruise ship, was carrying more than 6,000 passengers and crew on a week-long journey around the Caribbean when a guest tested positive, prompting wider contact tracing, according to Royal Caribbean. Cruise ships had been touted as the one of the “safest” vacations available back in summer 2021 when the cruise industry restarted in the US with new Covid protocols, following an extensive pandemic shutdown.

The Symphony of the Seas  was carrying 6,091 passengers and crew members. In a statement, Royal Caribbean said that a guest tested positive during the voyage, and subsequent cases were detected following contact tracing. It said 95% on board were fully vaccinated. Of the people who’ve since tested positive, 98% were fully vaccinated. The total number of cases amounted to 0.78% of the on board population.

It’s not yet known whether the highly infectious Omicron variant of coronavirus, currently spreading rapidly around the world, was responsible for the cases detected. Royal Caribbean rules dictate that all travelers on board a ship aged 12 and above must be fully vaccinated, and test negative before departure. The cruise line says it “strongly recommends” guests receive a booster dose prior to sailing, but this is not currently mandated.