Four vaccinated adults and two unvaccinated minors tested positive for covid-19 during routine end-of-trip screening Thursday on Royal Caribbean International’s Adventure of the Seas.

All six passengers were quarantined right away, and their travel companions and close contacts tested negative. The passengers left the ship Friday with their travel groups in Freeport, Bahamas, and they were sent home on private flights, with transportation provided directly to their homes, according to the cruise line. USA Today’s Morgan Hines, who was on the ship, was the first to report the news.

The two minors, both asymptomatic, were in the same travel party, but not with any of the adults who tested positive. The four vaccinated adults were not traveling together. Three were asymptomatic and one had mild symptoms, Royal Caribbean spokeswoman Lyan Sierra-Caro said.

Sierra-Caro said the cruise line was not able to determine on board whether the passengers were infected with the fast-spreading delta variant.

“We are going to be doing additional tests so we can look into this,” she said in an email.

Health officials have been sounding alarms about the highly contagious variant, saying that vaccinated people who are infected might be able to transmit the virus as easily as the unvaccinated. The CDC recommended this week that even vaccinated people wear masks indoors in areas of high or substantial community transmission.

Adventure of the Seas, which left from Nassau on Saturday for a weeklong voyage, requires crew and passengers 16 and older to be fully vaccinated against the virus. That vaccine age requirement will change to 12 for cruises leaving Nassau starting Sunday.

According to Royal Caribbean’s health protocols, all passengers 2 years of age and older must present a negative PCR test no more than five days before arriving in the Bahamas, though that requirement will change Sunday to either a PCR or antigen test three days before sailing. Passengers get tested again before the cruise ends to meet government health requirements.

About 1,100 guests are on board Adventure of the Seas — roughly 35 percent of capacity — along with around 900 crew members. Before arriving in Freeport on Friday, the ship stopped at the cruise line’s private Bahamian island and in Cozumel, Mexico. The cruise will end Saturday in Nassau.

Adventure of the Seas started sailing from Nassau in early June, at a time when cruise ships were still banned from departing from U.S. ports. Royal Caribbean said later that month that two young unvaccinated passengers on the ship had tested positive for the virus.

Other isolated cases on cruises have emerged, including in vaccinated passengers. Cruise line executives have acknowledged that infections are impossible to rule out but say the important thing is to keep them from becoming outbreaks. The industry shut down globally in March of 2020, and while cruises restarted earlier in some parts of the world, ships only started sailing again regularly from North America in recent months. A small number now are sailing from states including Florida, Texas and Washington, with more set to join in August.

The Centers for Disease Control and Prevention is tracking 64 ships that are sailing under its jurisdiction, either with passengers or with crew only. Of those, 21 have recently reported covid cases, according to the CDC, based on surveillance data from the past seven days.

The CDC recommended that people who are not fully vaccinated avoid cruise travel.

“Since the virus spreads more easily between people in close quarters aboard ships, the chance of getting COVID-19 on cruise ships is high," the agency advised. “It is especially important that people who are not fully vaccinated and at an increased risk of severe illness avoid travel on cruise ships, including river cruises.”

In response to skyrocketing covid cases, Royal Caribbean announced Thursday that all passengers older than 2 sailing out of the United States on trips of five nights or longer — regardless of vaccine status — will need to provide a negative test taken within three days of boarding. That rule applies to all cruises between Saturday and Aug. 31. Previously, only unvaccinated passengers needed proof of a negative test before boarding at U.S. ports.

Royal Caribbean International CEO Michael Bayley said on his Facebook page Friday that the change in policy is due to the fast spread of the delta variant.

“Yes, even with the vast majority of our onboard population highly vaccinated we are seeing more covid positive cases with vaccinated guests,” Bayley wrote.

The Miami-based cruise line described the new requirement as “an additional layer of precaution to ensure the safety of everyone onboard" and said the operator would continue to adjust its protocols as the public health situation evolves.

“We realize this will not make many guests happy just as it will comfort many guests,” Bayley wrote. “We are trying our very best to provide a safe and healthy and fun vacation for all our guests our crew and the communities we visit during these challenging times.”