One of the most well-known riverboats that cruise the Mississippi River is under investigation after an outbreak of COVID-19 last month during a cruise that made at least one stop at a Mississippi destination.

The CDC’s website shows the American Queen steamboat was investigated and is under observation after the outbreak.

Nola.com reported an outbreak that occurred last month of COVID-19 on a cruise that left New Orleans. The boats typically stop in Natchez, Mississippi, and Vicksburg, Mississippi, but apparently some of the regular stops were canceled. Passengers confirmed the boat stopped in Natchez, the website reported.

The newspaper reported that 11 people in a Delaware group of 24 contracted the virus, despite all passengers being required to be vaccinated. In total, approximately 200 passengers were aboard the boat. It’s unclear exactly how many passengers may have ultimately contracted the virus.

Cruise ships have been subjects of much debate relative to their safety during a pandemic.

“They’re big floating incubators,” Tulane University epidemiologist Susan Hassig told nola.com. “If anything is going to go wrong, it’s going to go wrong on a cruise ship.”