A new report from the Centers for Disease Control and Prevention (CDC) suggests thousands of airline passengers may have been exposed to coronavirus on flights.

According to The Washington Post, around 11,000 passengers may have been exposed to COVID-19, as the CDC confirmed at least 1,600 cases of people flying while contagious with the virus.

The CDC was not able to confirm any cases of viral transmission on a plane in the United States, though. CDC spokeswoman Caitlin Shockey told the Washington Post that “an absence of cases identified or reported is not evidence that there were no cases.”

“CDC is not able to definitively determine that potential cases were associated (or not) with exposure in the air cabin or through air travel given the numerous opportunities for potential exposure associated with the entire travel journey and widespread global distribution of the virus,” Shockey continued.

Previous studies published by the CDC found the coronavirus may have also spread on planes flying in other countries, but the organization maintains viruses do not spread as easily on flights due to air circulation and filtration.

One of the most recent reports detailed a Vietnam Airlines passenger who likely spread the virus to 14 of 217 fellow travelers on a March 1 flight from London to Hanoi. Despite the relatively low risk, the CDC still claims it is still possible to contract COVID-19 on crowded flights if there is a sick passenger on board.