Three recently published studies document instances of Covid-19 transmission on aircraft during the early stages of the pandemic.
The most concerning of the studies, which was peer reviewed and published in the CDC's Emerging Infectious Diseases journal, involved a March 10 Vietnam Airlines flight from London to Hanoi during which one passenger carrying Covid-19 was determined by the authors to have infected 15 other flyers, including 12 of the 20 other passengers who were sitting in business class.
The authors, who included university and public health scientists in Vietnam and Australia, reached their findings by interviewing and conducting Covid-19 tests on all 16 of the flight's crew as well as on 168 of the 201 passengers on the flight.
The authors note that the flight took place prior to the institution of mask mandates, which are now largely in place on aircraft around the world.
Still, they say that their findings raise alarms about the possibility of long-haul flights being superspreaders of Covid-19. The London-Hanoi flight in question lasted 10 hours.
The authors said their concerns are amplified by the fact that the spread occurred in the business-class cabin, where passengers are much more widely spaced than in economy cabins.
"We conclude that the risk for on-board transmission of [Covid-19] during long flights is real and has the potential to cause Covid-19 clusters of substantial size, even in business-classlike settings with spacious seating arrangements well beyond the established distance used to define close contact on airplanes," they wrote. "As long as Covid-19 presents a global pandemic threat in the absence of a good point-of-care test, better on-board infection prevention measures and arrival screening procedures are needed to make flying safe."
In a second study, also published in Emerging Infectious Diseases, a team of researchers from Hong Kong, England and Tanzania used genetic sequencing to confirm that two business-class flight attendants contracted Covid-19 on a March 9 flight from one or both members of a married couple who were symptomatic for the virus during the flight. The flight traveled between Boston and Hong Kong, a 15-hour route, and was operated with a Boeing 777 that could seat up to 294 passengers.
No other Covid-19 cases have been associated with the flight. The authors write that the findings are significant because previous reports of probable in-flight Covid-19 transmission lack genetic evidence.
A third study, published last week by a team of public health researchers in Greece, found less-startling results than the study of the Vietnam Airlines flight.
The authors investigated 18 international flights departing from or arriving in Greece between Feb. 26 and March 9. The flights contained a combined 2,224 passengers and 110 crew members. On the flights were a total of 21 people who later were determined to be positive at the time for Covid-19.
The authors contact traced 891 passengers and crew members who were within two meters of the infected passengers. They found five cases of probable in-flight transmission, four to passengers and one to a flight attendant. All five transmissions occurred on the same two-hour flight from Israel to Greece, on which two passengers had Covid-19.
The study, which appeared in the peer-reviewed journal Travel Medicine and Infectious Disease, doesn't address whether passengers were masked. However, the flights under review were operated before mask-wearing became common on most European flights.
"Our extensive investigation revealed that in-flight transmission of [Covid-19] most probably occurred and contributed to the importation of the virus in Greece," the authors concluded.
Airlines around the world have stepped up safety measures in the six months since Covid-19 was declared a pandemic. Carriers, aware of the importance of consumer confidence, have also stepped up marketing efforts to communicate their adoption of measures such as enhanced cleaning, mandatory mask-wearing, pre-flight health declarations and more.
Some countries have also begun requiring pre- or post-flight Covid-19 testing for international travelers.