A cruise ship returns to Miami with 48 coronavirus cases.

Dozens of passengers and crew members aboard a Royal Caribbean cruise ship tested positive for the coronavirus last week, according to the cruise line.

Royal Caribbean International said in a statement that the ship, the Symphony of the Seas, was carrying more than 6,000 guests and crew members when 48 people onboard tested positive.

The ship departed on Dec. 11 for a seven-night Caribbean cruise and returned to port in Miami on Saturday, the company said.

The cases were identified because of contact tracing after a guest tested positive, the cruise line said.

“Each person quickly went into quarantine,” the statement said. “Everyone who tested positive was asymptomatic or mildly symptomatic, and we continuously monitored their health.”

The cruise line said that the ship sailed with 95 percent of its guests aboard fully vaccinated and that 98 percent of the people who tested positive were fully vaccinated. It was not immediately clear whether any of the people who tested positive had received a booster shot of a Covid-19 vaccine.

Royal Caribbean requires travelers 12 and older to be fully vaccinated against Covid-19 and test negative before boarding the ship. Children who are not yet vaccinated must provide a negative P.C.R. test result before sailing and test negative at the terminal before boarding.

The company said that all its crew members were full vaccinated against Covid-19 and were tested weekly.

During the early days of the pandemic in 2020, outbreaks on cruise ships sickened hundreds of people and upended the tourism industry, as officials and companies struggled to keep crews and passengers safe.

For months, cruise ships were barred from sailing to many ports. Even after vaccinations became more widely available in the United States in April, allowing much of the travel industry to ramp up again, cruise ships remained docked in ports, costing the industry billions of dollars each month.

The cruise industry rebounded in the summer, with many companies reporting an increase in bookings and a better handle on Covid-19 protocols.

But a recent spike in coronavirus cases in parts of the United States and Europe and growing concerns over the Omicron variant may hinder that momentum.

This month, at least 17 coronavirus cases were identified on a Norwegian Breakaway cruise ship that docked in New Orleans, including a case of the new Omicron variant.