Corrections & clarifications: This story has been updated to reflect that though Celebrity Cruises has billed the Celebrity Millennium sailing as "fully vaccinated," some unvaccinated children are aboard the ship.
Two passengers from the United States who shared a room on board Celebrity Cruises' Celebrity Millennium ship, which was billed as carrying "fully vaccinated crew and guests", have tested positive for COVID-19, the cruise line said in a statement.
The tests came back positive during end-of-cruise testing, which is completed 72 hours before passengers return to the U.S., Celebrity Cruises spokesperson Jonathon Fishman told USA TODAY.
All adult passengers were required to show proof of vaccination in addition to a negative COVID-19 test within a 72-hour period before the ship departed from the Caribbean island of St. Maarten Saturday.
Children under the age of 12 are not yet eligible for COVID-19 vaccinations in the U.S., where the majority of passengers are from. Fishman noted there were fewer than 10 children on board and said no other vaccine exemptions had been granted.
There are more than 1,200 people on board the ship.
To enter St. Maarten, adult passengers had to fill out a form and provide a vaccination card, Fishman said. Cards were also checked at the port, he continued.
It’s still possible to test positive for the coronavirus even after getting vaccinated, experts say. The Centers for Disease Control and Prevention has said it can take weeks for a person’s body to build up immunity after getting vaccinated.
While the sailing took place outside of U.S. waters, meaning that the CDC's regulations did not apply, CDC spokesperson Caitlin Shockey told USA TODAY in a statement Friday that "cruising safely and responsibly during a global pandemic is very challenging," when asked about the emergence of COVID on board Celebrity Millennium.
The CDC's Framework for Conditional Sailing Order aims to mitigate that challenge by requiring a phased-in approach to U.S. cruising due to the continued spread of the pandemic globally. Shockey noted that the health agency's restrictions are place due to a "risk of resurgence in countries that have suppressed transmission, ongoing concerns related to restarting of cruising internationally, and need for additional time for the cruise industry to test the effectiveness of measures to control potential COVID-19 transmission on board cruise ships with passengers without burdening public health."
It's not possible for cruising to be a "zero-risk activity" in terms of COVID-19 spread, she added, noting that vaccines play a "critical role" in the safe resumption of operations. "CDC recommends that all travelers (passengers and crew) get a COVID-19 vaccine."
The Celebrity Millennium ship was docked in Curaço, and the passengers who tested positive remained on board in isolation, Fishman said Thursday. The ship will return to St. Maarten Saturday to disembark.
"The individuals are asymptomatic and currently in isolation and being monitored by our medical team," Celebrity's statement read Thursday. "We are conducting contact tracing, expediting testing for all close contacts and closely monitoring the situation."
Close contacts were confirmed negative after being tested again for COVID-19, Fishman told USA TODAY Friday.
This article originally appeared on USA TODAY: Passengers positive for COVID on cruise billed as 'fully vaccinated'