SAN JOSE, Calif. — How risky is it to fly during the coronavirus pandemic?
For clues, consider the travel histories of two of the country’s top infectious disease experts, each with parents on the other side of the country. One hasn’t flown since January when the new coronavirus was just emerging as a global threat.
The other just flew back to San Francisco after visiting his 90-year-old father in Florida last month —wearing a face shield and removing his medical-grade N-95 respirator mask for just 30 seconds to chug some water and pretzels —and “felt pretty safe” to see everyone else wearing masks as well.
“If the virus was able to get to me through that, it deserves it,” said Dr. Bob Wachter, chair of UC San Francisco’s Department of Medicine. “I was being as careful as I could.”
Major U.S. airlines say there has not been a single confirmed case of coronavirus transmitted on a domestic flight. A pair of recent studies, however, shed new light on the potential for a superspreading disaster aboard aircraft, even though the findings were based on long, overseas flights back in the spring before mask-wearing was widespread.
Both Wachter and Dr. Henry Wu, a senior physician at the Emory University School of Medicine in Atlanta, have yet to see any studies documenting airplane outbreaks when everybody was wearing a mask. But while Wu, who misses his parents who live in Hawaii, wouldn’t rule out flying, he hasn’t returned to the skies yet.
“Not that I wouldn’t,” said Wu, also director of the Emory TravelWell Center, who specializes in infectious disease and travel medicine. “Colleagues I hear from say the airlines are taking aggressive measures. But my threshold for traveling is pretty high right now.”
While the U.S. Centers for Disease Control and Prevention has said air travel can increase the risk of getting COVID-19 because it involves spending lots of time in crowded places, the agency also has said viruses don’t spread easily on flights because of how planes filter and circulate air. And until recently, there has been little research showing the coronavirus spreads aboard planes.
But one newly released study found that at least a dozen passengers became infected on a March flight from London to Hanoi while sitting near a young businesswoman believed to have had COVID-19. The other found that two flight attendants on a Boston to Hong Kong flight got the same genetic strain of the virus as a middle-aged couple on the plane who became sick with COVID-19 the day after they landed. The authors at the London School of Hygiene and Tropical Medicine and the University of Hong Kong concluded that the flight attendants got the virus from the infected passengers.
Wu said the London-Hanoi case was stunning for the sheer number infected by a single sick passenger, while also providing new evidence of how proximity affects the spread on an aircraft. The Boston-Hong Kong case was the first to conclusively demonstrate transmission on a plane through genetics.
“These studies are important because they add a layer of evidence that previous studies and reports have not had,” Wu said.
With what epidemiologists understand now about how the virus spreads, the studied flights in March were seemingly an ideal environment for it to find new hosts with hundreds of strangers seated close together for 10 to 15 hours before mask-wearing was prevalent.
“It’s clear if people were not wearing masks and were in a metal tube for 12 hours the possibility of spread is there,” Wachter said.
The International Air Transport Association, which represents the airline industry, said in an August report that the London-Hanoi flight was the only case it was aware of at the time documenting likely on-board transmission to a number of people.
The IATA said other examples suggest minimal risk, such as a Jan. 24 flight from Singapore to Hangzhou, China, in which 16 of the 335 passengers who were isolated and monitored for two weeks afterward tested positive, but just one was determined to have been infected during the flight.
Airlines for America, which represents major carriers including American, United, Delta, Southwest, Alaska, Hawaiian and Air Canada, said U.S. airlines have been implementing multiple layers of measures to help protect travelers.
But while airlines require masks, make passengers sign health declarations before boarding, regularly sanitize cabins, and equip planes with hospital-grade air filtration, they have resisted a suggestion many health and consumer advocates have urged — leaving middle seats empty to allow more passenger spacing.
That would come at enormous cost to airlines, which the IATA says are already nosediving, with August flights down more than half from a year earlier and losses totaling more than $84 billion.
The August IATA report said “alternate seat blocking does not achieve the required physical distancing” to be effective, and that achieving a three- to six-foot distance between passengers would be “uneconomical.”
The London-Hanoi flight study authors, led by an epidemiologist at Vietnam’s Department of Communicable Diseases Control, said their research suggests leaving middle seats empty is needed to reduce virus spread.
That study found that the 27-year-old Vietnamese businesswoman began feeling ill before boarding the 10-hour March 1 flight from London to Hanoi, where she sat frequently coughing by a window in the business class, where seats are bigger and spaced more than in economy. She tested positive four days after landing.
Of the 217 passengers and crew, 15 others later tested positive, 12 of them in business class, all but one of whom sat near the ailing businesswoman.
The large number of other passengers infected who were seated nearby surprised Wu, who called it a “worst-case scenario,” and other health experts.
The couple in the Boston-Hong Kong study, a 58-year-old man and his 61-year-old wife, also sat in business class for the 15-hour flight and were found to have infected a 25-year-old flight attendant who had served them in the section, as well as a 51-year-old flight attendant.
The long duration of the flights also was likely a factor in the virus’s transmission to other passengers, and Wu said the risk would be about a tenth as much on a one-hour flight as a 10-hour flight.
“It seems like catching it on an airline is a very, very low probability event,” Wachter said, “but probably not a zero probability event.”