Two young unvaccinated passengers on a Royal Caribbean International cruise out of the Bahamas tested positive for the coronavirus, the cruise line said.

The passengers, who were younger than 16 and traveling in the same group, left Adventure of the Seas before the end of the cruise on Thursday in Freeport with their companions. They returned home to Florida on a private flight arranged by the cruise company, CEO Michael Bayley said in a Facebook post.

It was the latest reminder of the difficulty of keeping the virus off cruise ships — and the latest test of the protocols meant to keep covid from spreading on board. Cruising from the United States on large vessels has been shut down since March 2020, but the first voyages are set to begin as soon as this weekend.

Sailings aimed at North Americans have also been leaving the Bahamas and other Caribbean destinations. Two passengers on one of those cruises — with the same vaccination rules — tested positive earlier this month. And a Royal Caribbean cruise that was scheduled to depart from Florida in early July was postponed after eight crew members tested positive before they were fully vaccinated.

The latest cases emerged near the end of a seven-night sailing on Adventure of the Seas, which left Nassau on Saturday with about 1,000 guests on board and stopped at the line’s private island in the Bahamas and Cozumel before reaching Freeport. All crew and passengers 16 and older are required to be vaccinated for those cruises; the cruise line said it could not provide the exact ages of the passengers who tested positive. Starting Aug. 1, all passengers 12 or older will have to be vaccinated.

Royal Caribbean and Carnival said on June 11 that plans to relaunch cruises from the United States were on track, despite infections detected on a ship. (Reuters)

Royal Caribbean spokeswoman Lyan Sierra-Caro said 92 percent of the people on board are fully vaccinated, and the other 8 percent are younger than 16.

Both of the young passengers tested positive Wednesday during regular end-of-trip testing and were quarantined right away. One was asymptomatic and one had mild symptoms. The cruise line said close contacts were identified and tested, and all were negative. Other members of the passengers’ immediate group were vaccinated and also tested negative.

According to the cruise line’s protocol, unvaccinated guests sailing on Adventure of the Seas need a negative PCR test no more than five days before arriving in the Bahamas and another test before boarding. Everyone is tested near the end of the cruise, which meets Bahamian requirements for unvaccinated travelers as well as U.S. requirements for anyone flying into the country.

During the cruise, unvaccinated passengers have to wear masks in all indoor public spaces unless they’re eating or drinking. Masks are not required in outdoor sections on the ship unless the area is crowded. Adventure of the Seas resumed cruises in the Bahamas on June 12.

“The return of Adventure of the Seas marks a start in the tremendous step forward our guests have been waiting for and we’ve been working toward for more than 15 months,” Bayley said in a statement at the time. “ … This is just the beginning, as we get ready to set sail from the U.S. for the first time on July 2.”