At least 48 passengers and crew members on Royal Caribbean's Symphony of the Seas -- the world's largest cruise ship -- have tested positive for COVID-19 and have been quarantined, the company said Monday.

Of the 48 people who tested positive during the sailing, 98 percent were fully vaccinated. That is not surprising, since the overall vaccination rate among the nearly 6,100 passengers and crew was already at 95 percent, according to Royal Caribbean, CBS News reported. Four of the 48 people who tested positive were close contacts of six passengers who disembarked earlier in the voyage, the cruise line said.

All travelers older than 12 years of age had to be fully vaccinated and produce a negative test result before the voyage, according to the company. Cruise companies tightened their COVID-19 protocols this summer -- requiring precruise tests for everyone, not just unvaccinated passengers -- when the delta variant was surging, The Washington Post reported.

The U.S. Centers for Disease Control and Prevention said it was aware of the Symphony of the Seas outbreak and was working with Royal Caribbean to "gather more information about the cases and possible exposures." Officials said specimens would be taken for genetic sequencing, according to The Post. The CDC reported last month that cruise companies had reported 1,359 cases -- many involving fully vaccinated crew or passengers -- on ships sailing from the United States between June 26 and Oct. 21. During that time, there were 49 hospitalizations and 38 medical evacuations due to COVID-19 or COVID-19-like illnesses, with one person dying after testing positive during a cruise.

Royal Caribbean said that those who tested positive on the Symphony of the Seas cruise were immediately isolated to prevent the spread of infection. The cruise line said they were either asymptomatic or experiencing mild symptoms, The Post reported.

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