Major airlines have said passengers are at low risk of contracting the novel coronavirus while flying, but some new studies suggest that the virus can—and has—spread on planes.

The germs on a plane—and how to avoid them

Research suggests coronavirus can spread on planes

Public guidance from CDC states that air filtration systems on airplanes makes it difficult for most viruses to spread easily during a flight. Like hospitals, airplanes typically are equipped with high-grade HEPA filters that fully and frequently filter the air.

However, CDC's guidance also emphasizes that air travel usually involves being in close contact with other people for long periods of time and encountering frequently touched surfaces—scenarios that have been associated with the novel coronavirus's spread. According to the Washington Post, CDC has identified at least 11,000 people who may have been exposed to the novel coronavirus during flights, though the agency cautioned that, due to limited contact tracing, it could not confirm whether any transmission of the virus occurred in those instances. Still, "[a]n absence of cases identified or reported is not evidence that there were no cases," Caitlin Shockey, a spokesperson for CDC's Division of Global Migration and Quarantine, said.

And four recent studies suggest the coronavirus can be—and has been—spread on airplanes.

For one of the studies, researchers from Vietnam's National Institute of Hygiene and Epidemiology examined a cluster of novel coronavirus cases that emerged among passengers on a 10-hour commercial flight from London to Vietnam. The researchers ultimately determined that a 27-year-old woman on the flight was the primary source of the outbreak.

According to the researchers, the woman had developed a sore throat and cough the day before her flight and continued experiencing those symptoms during the flight. Upon her arrival in Vietnam, she developed a fever and fatigue, and she began having difficulty breathing. Five days later, the woman was diagnosed with Covid-19, the disease caused by the novel coronavirus.

The woman was the only passenger on the flight who was experiencing symptoms of Covid-19. The researchers determined that the woman infected 12 other people on the flight, most likely through aerosol or droplet transmission. The researchers wrote, "We found no strong evidence supporting alternative transmission scenarios," so they concluded that "[i]n-flight transmission that probably originated from one symptomatic passenger caused a large cluster of cases during [the] long flight."

The researchers said they did not have data on whether people on the flight were wearing face masks or coverings, which could have helped to limit the virus's spread. However, they noted that the flight took place on March 1, before many airlines required individuals on planes to wear face masks or coverings.

For another study, researchers from South Korea and Harvard Business School tracked 310 passengers who were on an 11-hour evacuation flight from Northern Italy to South Korea. According to the researchers, passengers on the flight were screened for Covid-19 symptoms before boarding the flight. Passengers were kept at least six feet apart while waiting to board the plane, and most of the passengers wore N95 respirators during the flight except during mealtimes and when using the restroom. The passengers were immediately quarantined and isolated from each other for 14 days upon their arrival in South Korea.

The researchers reported that a 28-year-old woman who was on the flight started feeling ill eight days after the flight landed, and she tested positive for the coronavirus on her 14th day of quarantine. According to the researchers, the woman had quarantined for three weeks before the flight and wore an N95 mask during the flight except when using the restroom.

The researchers noted that another passenger who was on the flight tested positive for the coronavirus during the quarantine period but had not exhibited symptoms of Covid-19 during the flight. That passenger was seated three rows away from the 28-year-old woman who had become ill. Ultimately, the researchers concluded that, because the woman "did not go outside and had self-quarantined for three weeks alone at her home in Italy before the flight and did not use public transportation to get to the airport, it is highly likely that her infection was transmitted in the flight via indirect contact with an asymptomatic patient."

For a separate study published last month in JAMA Network Open, researchers investigated coronavirus cases among passengers who were on a flight from Israel to Germany that carried tourists who had been exposed to a hotel manager who was diagnosed with the virus. The researchers determined that seven of the passengers had been unknowingly infected with the novel coronavirus when they boarded the plane. The passengers tested positive for the virus on tests performed upon their landing in Germany. The researchers noted that "no measures to prevent transmission (eg, wearing of masks) had been applied" during the nearly five-hour flight, and they concluded that those passengers likely transmitted the virus two other people onboard the plane.

The researchers wrote, "The airflow in the cabin from the ceiling to the floor and from the front to the rear may have been associated with a reduced transmission rate," and "[i]t could be speculated that the rate may have been reduced further had the passengers worn masks."

And for the fourth study, researchers examined a cluster of coronavirus cases among four people, including two passengers and two crew members, who had been on a 15-hour commercial flight from Boston to Hong Kong on March 9 to March 10. The two passengers were a married couple who began experiencing symptoms of Covid-19 on March 10, were hospitalized for suspected Covid-19 on March 14, and tested positive for the coronavirus on March 15.

According to the researchers, one of the crew members involved in the study was identified through contact tracing "as a close contact of [those] patients." The crew member was informed that he may have been exposed to the virus and sought testing at an outpatient clinic on March 16. He tested positive for the coronavirus on March 17.

The other crew member involved in the study was a female attendant on the flight who developed symptoms of Covid-19 on March 18 and tested positive for the novel coronavirus on March 21. According to the researchers, "[t]here is no publicly available information of her travel history before the flight or her contacts with the other patients on or after the flight."

The researchers looked to determine whether there was transmission of the virus between the passengers and crew members by sequencing their viral genomes. The researchers explained that the "near full-length viral genomes from all four patients were 100% identical," meaning that transmission almost certainly occurred between the patients.

The takeaway? Airplane travel may be somewhat risky, experts say.

Michael Carome, director of health research at Public Citizen, said the research shows that flying is at least somewhat risky when it comes to coronavirus transmission, but he added that there are measures travelers can take to reduce that risk.

"There have been people who were infectious who traveled, and that means, indeed, there is real measurable risk of exposure on airliners," Carome said. However, he added, "[w]earing masks or face coverings is a simple, easy public health measure to take."

In addition, Carome said the aviation industry should look at adding more safety measures to mitigate the coronavirus's spread, such as temperature screenings and rapid coronavirus testing.

Joshua Santarpia, a microbiologist and pathologist at the University of Nebraska, said that, while research suggests transmission of the novel coronavirus is possible on planes, the risk of transmission on airplanes likely is lower than the risk of transmission in other crowded spaces. "If I were to pick between going into a crowded bar or getting on the airplane, I'd get on the plane any day," he said.

Likewise, Kayleigh Blaney, an epidemiologist for Michigan's Oakland County, said she's "exponentially more concerned with all the graduation parties, the fraternity and sorority parties happening on college campuses than I am with flying."

According to the Post, when asked about the recent findings, Katherine Estep, a spokesperson for Airlines for America, said there have been no documented cases of coronavirus transmission on U.S. flights. "Flying remains a safe and healthy experience," Estep said (Weixel, The Hill, 9/18; Duncan, Washington Post, 9/19; Kelleher, Forbes, 9/19; Cong Khanh et al., CDC's Emerging Infectious Diseases, 9/18; Hwan Bea et al., CDC's Emerging Infectious Diseases, 8/21; Hoehl, JAMA Network Open, 8/18; Choi et al., CDC's Emerging Infectious Diseases, 9/18).