Future Tense

What Do Two New Studies Really Tell Us About Coronavirus Transmission on Planes?

A woman wearing protective equipment, including a mask, stands in the aisle of an empty plane as she puts a bag into an overhead compartment.
A flight attendant at El Dorado International Airport on Aug. 31 in Bogotá, Colombia. Guillermo Legaria/Getty Images

This article is part of Viral Studies, a Slate series in which we break down recent viral articles and—most importantly—their caveats.

Air travel has long provoked anxiety in passengers, but even more so after SARS-CoV-2 has taken hold worldwide. In the past several months, scientists and public health experts have emphasized that the risk of contracting the virus is greatest in small, enclosed spaces with many people—and where do people feel more claustrophobic and crammed than an airplane?

So recently, two new case studies of potential coronavirus transmission aboard airplanes immediately made the rounds online, creating buzz about the “dangers” of in-flight COVID transmission. Several outlets uncritically reported the authors’ findings, both published in the U.S. Centers for Disease Control’s journal Emerging Infectious Diseases, which looked at two clusters of suspected coronavirus transmission on flights. While the possibility of plane transmission may sound alarming, neither of these studies tell us much about COVID-19 risks that we didn’t already know.

The first thing to note is that both cases took place on long flights in early March. One case study, led by researchers at Vietnam’s National Institute of Hygiene and Epidemiology, found a cluster of cases on a 10-hour flight from London to Hanoi, which landed March 2. A 27-year-old woman had a fever and was coughing during the flight, and the researchers later found 15 other people on the flight who tested positive for COVID-19. The other study, led by researchers in London and Hong Kong, found that on a 15-hour March 9 flight from Boston to Hong Kong, a couple sitting in business class infected two flight attendants.

The length and timing of these flights introduced potential risks that may not apply to current air travelers. Scientists have long emphasized exposure time as a risk factor; the longer you’re in close proximity to an infected person, the more likely you are to encounter the virus particles they’re shedding. But unlike the passengers in these clusters, most air travelers are not taking 10- to 15-hour flights.

And the timing of these flights —early March—means that they occurred before mask-wearing on planes was compulsory. That’s no longer the case; all major airlines now require passengers to mask up. (A colleague who flew recently told me that on her flight last week, the pilot did not mince words about “blacklisting” any uncooperative passengers.) “Wearing masks makes a huge difference,” says Qingyan Chen, professor of mechanical engineering at Purdue University and a former director of the FAA’s Airliner Cabin Environment Research Center. Lab studies have shown that masks of all materials can be effective at filtering virus particles and decrease the number of particles people expel into the air—all important factors to consider when breathing and talking in close proximity to other plane passengers. Hopefully, the adoption of mask-wearing has prevented more of these clusters from popping up.

While it’s likely these two case studies identified cases in which COVID-19 was transmitted aboard planes, the exact mechanisms of transmissions are not crystal clear. In the Boston-to-Hong Kong case, researchers performed genetic analyses of samples from all four infected people and found a 100 percent match, suggesting that the couple in business class passed on the same strain to their flight attendants. The genome sequencing is convincing, says Lin Chen, director of the Harvard-affiliated Mount Auburn Hospital and president of the International Society of Travel Medicine. But even so, there’s still the possibility that the crew could’ve happened to be exposed to the same strain somewhere besides the flight. With the London-to-Hanoi study, Chen says she also suspects plane transmission was likely, but the causality is even harder to discern, because the researchers did not sequence samples. “Until genome sequencing is done on a lot of these suspected in-flight transmission cases, it’s hard to say for sure.”

Joseph Allen, a researcher at the Harvard T.H. Chan School of Public Health, laid out his doubts about the case study’s causality in a Twitter thread. He points out that based on the timing of some passengers’ symptom onset and what they did immediately after the flight (a few went on cruises!), it’s possible—or even likely—that some of these “flight-associated” cases actually contracted the coronavirus elsewhere, not necessarily on the flight itself.

That’s not to downplay the risks of getting COVID-19 on a plane; it can certainly happen. But the airplane itself is not particularly dangerous, says Purdue’s Chen. In most large airplanes, air circulates through the cabin in five to seven minutes, and then is filtered through a HEPA filter that is 99.97 percent effective at filtering small particles. “The air supply into the cabin is super clean, but that doesn’t mean you have zero risks there,” he says. The biggest risk, as he sees it, is in the moments when passengers might take masks off—like while eating. Chen recommends staggering snack times so that passengers don’t all have their masks off at the same time. “The risk occurs during this period because I talk, I generate droplets, and that goes directly to the people sitting next to me,” he says.

Harvard’s Chen says these studies are helpful confirmation that flight transmission can happen, but agrees that overall, the risk is generally low. First, she points to the number of infected people on these planes: Both carried more than 200 passengers, and the number of potentially infected passengers made up just a small portion of people aboard. And, she says, the fact that few clusters have been reported should be heartening, as well as the reports of cases in which infected people have boarded an airplane without infecting anyone.

Again, that doesn’t mean that air travel is safe. In the U.S., in particular, an extremely high base rate of the virus paired with a shortage of testing, contact tracing, and travel regulations means that any clusters may be harder to track. (Hong Kong and Vietnam have both been lauded for their post-flight procedures and contact tracing, which made these detailed case studies possible.) Additionally, people may be attuned to the risks of transmission while inside the airplane, but if you’re concerned about travel, it’s important to consider risky situations you encounter outside the metal tube, too. Consider the poorly ventilated small trams within the airport, or how close you might get to others waiting to board or deplane, or the people you’ll encounter traveling to and from the airport. Whether you’re on an airplane, traveling to the airport, or just sitting at your neighborhood bar, COVID-19 risk exists, and these studies shouldn’t change the individual behaviors we undertake to minimize that risk: Avoid unnecessary travel, wear a mask, wash your hands well, and stay home if you’re sick.

Future Tense is a partnership of Slate, New America, and Arizona State University that examines emerging technologies, public policy, and society.