Almost 10,000 people tested negative before flying. Just one was positive after landing, a study shows.

·4 min read

A study of Italy-bound Delta Air Lines passengers found that a mandatory PCR coronavirus test taken within three days of flying weeded out the vast majority of covid-infected travelers.

The study examined data from the airline's program that allowed travelers to avoid quarantine in Italy if they provided proof of a negative molecular test within 72 hours and got a rapid test at the airport in Atlanta or New York before departing. Passengers had to undergo another rapid test after landing in Italy.

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The study was conducted by Mayo Clinic, Delta Air Lines and the Georgia Department of Health. The article, which published earlier this month, will appear in the journal Mayo Clinic Proceedings.

Of 9,853 travelers who got a negative coronavirus test within 72 hours of departure, .05 percent - or five people - were discovered to have an active infection either immediately before or after the flight. The flights took place between December 2020 and May 2021; the study says the average community infection prevalence rate was estimated at 1.1 percent during that time.

Four of those five passengers were discovered by rapid antigen tests, which were confirmed by rapid molecular tests, before boarding at the airport. They were not allowed to fly.

That left one passenger who did not test positive until arriving in Italy.

"These data suggest that even at this higher level of active community infection, a single molecular test performed within 72 hours of departure can decrease the rate of active infection on board a commercial aircraft to a level that is several orders of magnitude below active community infection rates," the study says. "The addition of other interventions, including universal masking at the airport and onboard aircraft, increase in frequency of air exchanges and enhanced clearing, physical distancing during deplaning activities, increasing vaccination rates among travelers and exclusion of symptomatic individuals, further enhances safety."

Delta said it covered the costs of the testing associated with the flights but had no funding role in the study. Mayo Clinic worked with Delta to design the testing program as part of a long-standing partnership.

In an article on its news page, Delta said the results show "the feasibility of putting in place a testing protocol with meaningful impact."

"We are going to live with covid-19 variants for some time," Henry Ting, the airline's chief health officer, said in the piece. "This real-world data - not simulation models - is what governments around the world can use as a blueprint for requiring vaccinations and testing instead of quarantines to re-open borders for international travel."

Many international destinations require testing before passengers depart, sometimes in addition to vaccination. But with the exception of flying to Hawaii, there are no mandates for testing in advance of domestic flights.

"If you're in a confined space for six-plus hours as an international flight entails, that's going to inherently present more risk than an hour-long flight domestically," said lead author Aaron Tande, an infectious-diseases physician at Mayo Clinic in Rochester, Minn. He said there are larger discussions to have about whether tests are needed for all flights as opposed to just those that carry higher risk.

The study was conducted before the extremely contagious delta variant become dominant in the United States, and included many months when vaccines were not available to the general population.

Tande said the absolute numbers in the research might differ if it were repeated now, with the delta variant at play, more people vaccinated and a different level of community infection. Still, he said, the results show that testing would decrease the risk of infection on a plane.

Tande said it's still clear that air travel needs "multiple layers of mitigation." He said most reported cases of transmission during flights happened either before masking was universal or when there were lapses in mask-wearing, which can include eating and drinking.

"To me, it's very clear you should have universal masking on flights," he said. "And then testing is another layer to decrease risk."

He said his ideal safe flight would take an extra step: Everyone on board would be vaccinated, tested and masked.

"Do I want to be on a vaccinated flight?" he said. "100 percent."

The study acknowledges "important limitations," including the chance that those who were flying were more likely to consider themselves at low risk of getting the virus, or that travelers with exposure might have decided to stay home because they knew they would be tested.

"This possibility may limit the generalizability of our findings and recommendations to the overall population of commercial air travelers," the study says.

Rapid antigen tests, which are less sensitive than molecular tests, also could have produced some false negatives and allowed infected people through the screening. Tande said the study did not follow those 9,800-plus passengers to find out if any of them - including those on the flight with the person who tested positive - became infected.

The article said the results show a "low yield" of additional rapid testing at the airport following a negative PCR test. According to the study, that suggests "this additional testing is unlikely to add safety alongside other mitigation efforts (i.e., masking) especially as vaccination rates are rapidly increasing."

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