Cruise ships were the site of a number of massive COVID-19 outbreaks at the start of the pandemic last year.

Now, even with strict COVID health and safety policies, it appears that two passengers have tested positive for the virus on a fully vaccinated Royal Caribbean ship, People reports. The Celebrity Millennium left St. Maarten on June 5 for a week-long jaunt around the Caribbean.

The cruise line has since issued a statement. “Today two guests sharing a stateroom onboard Celebrity Millennium tested positive for COVID-19 while conducting the required end of cruise testing,” it said. “The individuals are asymptomatic and currently in isolation and being monitored by our medical team.”

“We are conducting contact tracing, expediting testing for all close contacts, and closely monitoring the situation.”

The Celebrity Millennium has stern guidelines that passengers must meet. Those who are 15 and older who board the vessel have to be fully vaccinated two weeks ahead of when the ship is set to sail. After Aug. 1, that policy has been extended to anyone who is 12 and older.

For this trip, passengers had to prove that they’ve been vaccinated and provide a negative COVID test within 72 hours before the ship left St. Maarten. “Celebrity Millennium is sailing with the 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,” Royal Caribbean said. 

The statement continued, “This situation demonstrates that our rigorous health and safety protocols work to protect our crew, guests, and the communities we visit.”

The number of deaths from the virus has dropped as more people have gotten vaccinated, with this week marking the lowest number of COVID infections and deaths since last March.