Writing for Vox, Abraar Karan, a physician at Brigham and Women's Hospital and Harvard Medical School, describes a recent Covid-19 spreading event on an international flight from Dubai to New Zealand to assess whether flying is safe—and whether that's even the right question to ask.

See our latest resources and insights on Covid-19

An (unclear) case study

Karan writes that one of the most common questions patient ask is whether it is safe to fly. But the answer, Karan writes, is complex. To demonstrate, Karan cites a September 2020 flight from Dubai to New Zealand, in which four passengers contracted the novel coronavirus from one other passenger who boarded the plane without knowing he or she was already infected.

According to Karan, before delving into the details of the outbreak, it's critical "to understand that transmission doesn't just happen. It actually takes a number of protections falling apart one after the other," much like the "Swiss cheese model," in which one layer of protection cannot defend against infection, but layering on multiple layers of protection can.

With that in mind, Karan explains several flaws that may have contributed to the spreading event. For instance, the person who carried the infection onto the plane was incorrectly reported as having received a test within 48 hours of the flight—in reality, he or she had a four-day-old negative test result. In addition, only two of the four infected passengers said they wore masks during the flight, and the plane had to shut down its power unit for 30 minutes during a refueling—which meant the ventilation system was also shut down.

According to Karan, "[a]ll of these factors introduce a number of 'what ifs' that we can ask about what could have prevented the transmission—and whether improving these steps could make other flights safer." For example, if the originally infected passenger, or "index case," had taken a more recent test, or if the airline had used rapid antigen testing, it's "very possible" his or her infection would have been identified in time. (Karan adds here that the originally infected passenger didn't report any symptoms until two days after the flight—meaning symptom-based screenings measures would have been ineffective.)

And there are other potential considerations, Karen adds: What if the passengers—all of whom sat within four rows of the infected passenger—had been seated further away? Or what if the 18-hour flight had been shorter? Or if the ventilation system had never been shut off? Or if the passengers wore higher quality masks? 

As Karan points out, we ultimately "don't know for sure which of these things it was; more likely, it was a combination of all of them"—a realization that in turn demonstrates how difficult it can be to understand coronavirus transmission, whether on a flight or elsewhere.

'Does this mean don't fly?'

According to Karan, the flight in the case study was unique in that it landed in New Zealand, where passengers were subject to a mandatory 14-day quarantine period in government facilities and regular testing. As a result, researchers were able to identify the potential transmission and link it back to the flight via genomic studies.

But "[t]his level of follow-up is rarely happening in the United States," Karan writes, where post-flight quarantining and testing are on the honor system—making it "much harder to know if transmission happened on the flight or afterward." As a result, we don't really have a good grasp of how many infections are happening on U.S.-related flights, Karan writes.

"Does this mean that all flights are dangerous?" he asks. "Does this mean don't fly?"

According to Karan, the answer ultimately "depends on numerous protections holding up, which at times can and will be out of our control, both for flights and for all other activities that we partake in." Speaking for himself, Karan says he would advise against all unnecessary travel, not just because of what may happen on any given flight, but because of what happens when passengers disembark. As he puts it, "The more we move around and meet up with others, the more the virus spreads."

Overall, however, the takeaway is that "[w]e should … be increasing all prevention measures," Karan writes. "With new [novel coronavirus] variants, even our airplane travel will require that we do many things right to avoid one especially wrong outcome" (Karan, Vox, 1/25).