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Six passengers on Royal Caribbean's Adventure of the Seas ship have tested positive for COVID-19, the cruise line confirmed to PEOPLE on Friday.
Four of the guests, who are not part of the same party, have been vaccinated against the virus, according to a statement from Royal Caribbean. One person is experiencing mild symptoms while the other three remain asymptomatic.
The two other guests, who are in the same party, are unvaccinated minors. They remain asymptomatic.
The six guests were "immediately" quarantined and all close contacts, including those they were traveling with, have since tested negative.
The cruise, which departed from Nassau, Bahamas, on July 24, required travelers age 16 and older to be fully vaccinated against COVID-19 and return a negative test prior to boarding. Children ineligible for inoculation were required to return negative tests as well.
All crew members on the ship are vaccinated, according to the cruise line.
The six positive tests were returned during routine testing required of all Adventure guests before returning home.
"Each guest and their immediate travel parties disembarked in Freeport, The Bahamas, and separately traveled home via private transportation," Royal Caribbean said in the statement to PEOPLE.
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As of Friday, 49.4% (163.8 million) of the United States populace is fully vaccinated against COVID-19, according to data from the Centers for Disease Control and Prevention (CDC), while 57.2% (189.9 million) have received at least one dose.
But the new Delta variant, coupled with lingering vaccine hesitancy, is complicating the United States' recovery.
An internal memo from the CDC suggests the Delta variant can spread via vaccinated individuals, The Washington Post reported Thursday night. CDC Director Dr. Rochelle Walensky confirmed the document's authenticity to CNN on Friday.
"I think people need to understand that we're not crying wolf here. This is serious," she told CNN.
According to the memo, Delta can spread as easily as chickenpox. It also suggests fully vaccinated people might spread it at the same rate as unvaccinated people.
On Thursday, Walensky said the CDC would be altering its guidance for inoculated individuals in wake of the highly contagious variant. It will recommend people wear masks indoors -- including in schools -- where transmission of COVID-19 is sustained or high.