The Centers for Disease Control and Prevention has investigated 1,600 cases of people who flew while at risk of spreading the coronavirus, identifying nearly 11,000 people who potentially were exposed to the virus on flights.
But though the agency says some of those travelers subsequently fell ill, in the face of incomplete contact tracing information and a virus that incubates over several days, it has not been able to confirm a case of transmission on a plane.
That does not mean it hasn't happened, and recent scientific studies have documented likely cases of transmission on flights abroad.
"An absence of cases identified or reported is not evidence that there were no cases," said Caitlin Shockey, a spokeswoman for the CDC's Division of Global Migration and Quarantine.
"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 wrote in an email.
She said that though the agency has received information about people who may have been exposed on flights subsequently becoming ill with the novel coronavirus, pinpointing when someone was exposed is difficult. Local health authorities also might not be able to test people reported as exposed or share test results with the CDC, she said.
In guidance for the public, the CDC acknowledges that viruses do not spread easily on planes because of the way the air is filtered, but it also emphasizes that air travel means being in proximity to people for long periods and encountering frequently touched surfaces on planes and in airports.
The CDC's guidance for all kinds of travel remains that staying home is the best way to protect yourself and other people from the virus.
Michael Carome, the director of health research at the consumer organization Public Citizen, said the CDC numbers demonstrate that flying poses at least some risk of being exposed to the virus. Public Citizen has called on the Department of Transportation to mandate that masks be worn on planes.
"Wearing masks or face coverings is a simple, easy public health measure to take," Carome said. "There have been people who were infectious who traveled, and that means, indeed, there is real measurable risk of exposure on airliners."
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The total numbers of people flying while carrying the virus and those exposed are unknown.
States have long worked with the federal government to track the spread of infectious diseases on planes. But of the nearly 100 state and major local health departments contacted by The Washington Post, most did not provide a number of coronavirus cases they have documented involving air travel, with some saying they were not tracking that data. In addition, not every case identified locally becomes a CDC investigation.
Six health departments were able to provide numbers, saying they had identified more than 500 cases between them, figures that in some cases covered only a few weeks.
Despite the documented risk of exposure, some experts as well as state and local public health officials say being on a plane presents less of an infection risk than drinking at a crowded bar or going to an indoor party. The air on a plane is pulled out of the cabin and filtered, then mixed with fresh air and pumped back in. And airlines have imposed mask requirements even without a federal mandate.
Asked about the CDC's numbers, Katherine Estep, a spokeswoman for the industry organization Airlines for America, emphasized that there are no documented cases of transmission involving U.S. flights.
"Flying remains a safe and healthy experience," Estep said.
But though the CDC has not confirmed such transmissions domestically, new studies of flights in Asia and Europe have identified instances where scientists think the virus has spread on commercial flights - including one where passengers were wearing N95 masks, according to a paper published in a CDC journal. N95 masks, when worn correctly, are thought to offer some of the best protection against the virus.
Public health authorities in Britain recently ordered almost 200 passengers and crew into two weeks of quarantine after it was discovered that seven people traveling to Wales from the Greek island of Zante on Aug. 25 were infectious on the flight. Passengers told the BBC that boarding the plane was a free-for-all and that passengers were lax about wearing masks on the flight.
An analysis by the International Air Transport Association published in August identified four cases of possible transmission on aircraft. Among them was a March 2 flight from Britain to Vietnam on which one symptomatic passenger is likely to have transmitted the virus to 15 other people, according to a study published Friday in the CDC journal. Most of them were sitting close to the symptomatic passenger in business class, but people elsewhere on the plane also tested positive.
The study's authors said their results challenge the airline industry's safety claims, although they noted that the case dates to when wearing masks was not yet widespread.
"Our findings call for tightened screening and infection prevention measures by public health authorities, regulators, and the airline industry," they wrote, calling for mandatory mask usage, good hand-washing hygiene and systematic testing and quarantining of arriving passengers.
Two other scientific studies also have identified likely cases of transmission. One of those studies, also published in the CDC journal, looked at an evacuation flight from Northern Italy, one of the first regions in Europe to be badly hit by the virus, to South Korea. The flight was closely monitored by Korean health authorities, and passengers were given N95 masks to wear.
But a team of Korean scientists reported that testing once the flight landed revealed that the plane had carried six asymptomatic passengers. On the eighth day after the flight landed, a 28-year-old woman began to feel ill and ultimately tested positive for the novel coronavirus.
The woman had been self-quarantining before the flight and was in quarantine after arriving in South Korea, leading the researchers to conclude that she contracted the virus on the plane, perhaps when she removed her mask while using the lavatory.
The research team acknowledged the role of the plane's filters as a defense against the virus but said contaminated surfaces or the mingling of passengers during boarding could be opportunities for exposure.
"Our results suggest that stringent global regulations for the prevention of COVID-19 transmission on aircraft can prevent public health emergencies," the scientists concluded. They recommended the use of masks, hand washing and social distancing while getting on and off planes.
Another study examined a flight from Israel to Germany carrying tourists who were exposed to a hotel manager who had the virus. Seven of them were carrying the virus when they boarded the plane, and the study concluded that it was most likely transmitted to two more people on board.
"Transmissions do occur, even though the air circulation in the cabin likely reduces the rate of transmission," said Sandra Ciesek, a virologist at the University Hospital Frankfurtin Germany and one of the study's authors.
People on the flight were not wearing masks, a factor that Ciesek said could have made a difference. She pointed to another study in which she was involved concerning an evacuation flight from China to Germany with sick passengers. Those passengers wore masks, Ciesek said, and there were no cases of transmission.
Though the studies suggest transmission is possible on planes, Joshua Santarpia, a microbiologist and pathologist at the University of Nebraska who was not involved in the studies, said that if the same groups of people were put in other enclosed spaces for several hours, he would expect to see many more people falling ill.
"If I were to pick between going into a crowded bar or getting on the airplane, I'd get on the plane any day," said Santarpia, who said he has flown about two dozen times for work during the pandemic and took a plane trip with his son.
Convincing travelers that they will be safe on board has been a top priority for the airline industry, which has been among the hardest hit during the pandemic, with passenger numbers plummeting more than 95 percent. Passengers have slowly been returning, but the industry remains on its heels, warning of tens of thousands of furloughs and layoffs this fall.
Aviation companies are backing their own research to better understand the potential risks.
In August, an arm of the Defense Department that organizes commercial flights for military members, their families and contractors worked with Boeing and United Airlines to gather data on how the virus moves inside planes. The tests, conducted at Washington Dulles International Airport, involved the release of aerosol particles among mannequins simulating passengers. Santarpia is working on the study, which is expected to report results in October.
This month, Harvard's T.H. Chan School of Public Health announced the launch of anaviation industry-sponsored study on the risks of flying. Airlines have adopted new cleaning technologies and imposed some of the strictest mask requirements in the country, banning hundreds of passengers who have refused to comply.
But the measures are not perfect - stories of passengers refusing to don masks regularly circulate on social media - and scientists have long understood the potential for viruses to spread in aircraft cabins even with air filtration systems.
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Airlines also have no way to prevent people who are infected but asymptomatic from flying. And Kayleigh Blaney, an epidemiologist for Oakland County in the Detroit area, said people report flying despite having symptoms of covid-19 - the disease caused by the novel coronavirus - telling contact tracers: "Well I got sick while I was here and I had to get home."
Labor unions and Democrats in Congress continue to press for more safety measures. Carome, of Public Citizen, said the aviation industry could explore such options as temperature screenings and rapid testing, something airlines are interested in doing as trials as a way of restarting international travel.
Officials at state and county health departments said that, like the CDC, they're regularly documenting cases of people flying while capable of spreading the virus.
When local officials find a case, they typically share the information with their state's health authorities, and then the CDC coordinates with airlines to determine the identities of people who potentially have been exposed. The CDC's rules for determining exposure have evolved along with its understanding of the virus but include people sitting within six feet of a reported case or all passengers on flights without assigned seating.
Contact information is then passed back down the chain, but sometimes it is incomplete or out of date.
In 2018 and 2019, the CDC investigated about 150 cases of pathogens such as the measles virus and tuberculosis bacteria spreading on planes. Shockey, the CDC spokeswoman, said the team that handles those cases has been expanded to handle the increased workload of tracking coronavirus cases.
"This is an unprecedented disease outbreak response, and the team has been expanded and are working tirelessly to meet the demands," she said.
The relationship between airlines and the CDC has long been contentious. Early in the coronavirus outbreak, there was a dispute over a proposal to require better passenger contact information to be collected for international flights. The industry proposed developing an app that it would hand over to the government, but no final agreement has been reached despite the White House taking up the issue.
Judd Deere, a spokesman for President Trump, said the administration, "continues to work with the airlines on the best solution to protect the health and safety of the public not only during this ongoing pandemic but for future ones as well."
Most state and local health departments did not provide numbers on cases linked to air travel, with some saying they weren't systematically tracking them. The 500 cases The Post did document came from three counties, two states and the District of Columbia, and in some of the cases, the numbers represented cases for only a few weeks. The Salt Lake County health department alone has counted 275. Others said that even if they couldn't supply numbers, they regularly learned of cases of infection where people reported having traveled by air.
Air travel played a major role in spreading the virus around the world and the United States, but some health officials still say they consider exposure on planes a relatively minor risk. The acting state epidemiologist in Louisiana, Theresa Sokol,said officials there have not identified any coronavirus clusters involving air travel. In contrast, 41 outbreaks in the state have been ascribed to bars, 41 to restaurants and 25 to day-care centers.
The Vermont Health Department likewise said no one in the state who the CDC reported to have been exposed on a plane has become a coronavirus case.
Blaney, the Oakland County epidemiologist,said she also was not aware of any plane exposures turning into positive cases.
"I'm exponentially more concerned with all the graduation parties, the fraternity and sorority parties happening on college campuses than I am with flying," Blaney said.
But she advised people who fly to be vigilant about keeping their masks on and understanding airlines' safety protocols.
“A lot of it has to do with how safe you’re being while you’re flying,” Blaney said.