Two guests aboard the Celebrity Millennium tested positive for Covid-19 in required end-of-cruise testing, the cruise line said in a Thursday news release.
Celebrity Millennium -- the first major cruise ship allowing American passengers since the pandemic forced a pause on the industry -- set sail on Saturday.
The two positive guests, which shared a stateroom on the ship, are asymptomatic and are now in isolation while being monitored by a medical team, the release said.
"We are conducting contact tracing, expediting testing for all close contacts and closely monitoring the situation," the news release said.
The ship is sailing "with fully vaccinated crew and guests and following comprehensive protocols that align with our destination partners and exceed CDC guidelines to protect the health and safety of our guests," the release said.
The 7-day cruise from St. Maarten includes port stops in Barbados, Aruba and Curacao.
All guests were required to show vaccination proof and a negative COVID-19 test within 72 hours before from St. Maarten Saturday, the release said.
About 500 passengers are on board, over 95% who are fully vaccinated, according to previous CNN reporting. Children who could not yet receive vaccinations were required to show negative Covid-19 tests.
"This situation demonstrates that our rigorous health and safety protocols work to protect our crew, guests and the communities we visit," the news release said.
Details about the two passengers' Covid-19 vaccinations are unknown.
On Monday, the Royal Caribbean Group, the parent company of Celebrity Cruise Lines, said on Twitter that returning to Barbados "for the first time in over a year is truly special."
"Every measured step was taken to ensure that we protect the health and safety of our guests, crew and the residents of this treasured destination and we are thankful," the post said.
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CNN's Maggie Fox contributed to this report.