Nearly 10,000 flyers have undergone Covid testing – here’s how many infections got through

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A study conducted earlier this year suggests there may be a way to reduce the number of Covid infections aboard commercial aircraft to virtually zero.

The results of the study appeared in a peer-reviewed paper published Sept. 1 in the medical journal Mayo Clinic Proceedings. The paper – a joint effort of Mayo Clinic, the Georgia Department of Public Health and Delta Air Lines – showed that one polymerase chain reaction (PCR) test performed within 72 hours of flying reduced the number of infected travelers on board to 0. 0.05%. That’s five people for every 10,000 passengers.

At the time of the study, the infection rate in the US was 1.1% — or about 1 in every 100 people.

‘A damn low number’

The findings analyzed data from Delta’s preflight testing program that ran from December 2020 to May 2021.

Here’s how Delta’s testing program worked: Passengers on select flights from New York City and Atlanta could fly to Italy, without having to go into quarantine upon arrival, if they tested negative for Covid-19 via a PCR test within 72 hours of the flight , a rapid antigen test before take-off and a rapid antigen test on landing.

Of the 9,853 people who tested negative through the PCR test, four tested positive at the airport through rapid antigen testing. A rapid molecular test confirmed the diagnoses and these people were not allowed to fly.

Of the passengers flying to Italy, one tested positive on landing.

This translates to one case detection per 1,970 travelers “at a time of high prevalence of active infection in the United States,” according to the article.

“That’s a pretty low number,” said Dr. Aaron J. Tande, the paper’s lead author and infectious disease specialist at the Mayo Clinic in Rochester, Minnesota.

The study suggests that one PCR test within three days of flying largely obviates subsequent testing at the airport, especially when combined with on-board masking requirements and increasing vaccination coverage among pilots.

‘Limitations of the study

The journal article lists several “limitations” that may have influenced the results of the study, including the role preflight testing had on traveler behavior. Participants with suspected Covid infections may have chosen not to travel. Others may have been more diligent about wearing masks and self-isolating, knowing they had to test negative to fly, Tande said.

“I can’t say that made the positive test count so low — or was it really that the 72-hour test was that good,” he told CNBC. “But… the end result is that it’s a safer flight for people, and that’s what we want.”

Tande said the findings are based on the Covid-19 strains circulating in the United States in the first half of 2021, not the more contagious delta strain that now dominates.

“I don’t think you could say that if you repeated the study now — with a different percentage of community infection and a different virus — you’d get the exact same result,” he told CNBC. “I think you would see a significant decrease in the infection rate on board” compared to not testing at all.

Safer but less viable options

The pilot program considered five testing strategies, two of which may have detected even more infected flyers.

For example, a single rapid molecular test at the airport may have found more infections because it minimizes the time between testing and flying, and thus could catch infections that occur during that time. Adding a 72-hour PCR preflight test would likely yield even more, according to the study.

However, a preflight PCR test is the “better approach” because it is more feasible, Tande said. PCR tests are widely available, “more sensitive” — meaning they’re better at detecting positive cases — and they take the logistics of testing out of airports, he said. Pre-testing also gives infected travelers time to reorganize their plans, rather than surprise them just before their flights depart.

Test flights or vaccinated flights?

Preflight PCR testing may make flying safer, but most passengers now fly without it. And airlines are tight-lipped about making it mandatory going forward.

However, testing could become a de facto rule on international flights if arrival countries require passengers to enter. A spokesman for Delta Air Lines declined to say whether it would test its passengers, but said “each country’s government is responsible for setting their own requirements.”

Tande said he would feel safer taking a flight that required passengers to pass preflight PCR tests. But given the choice, he said he preferred a vaccine-only flight.

“I would definitely go for the vaccinated flight — and (I) mask,” he said.

Qantas CEO Alan Joyce last week said passengers on their international flights should be vaccinated, according to US officials are currently debating whether vaccinations are necessary to fly both domestically and internationally, as reported this week by The Washington Post.

“Unfortunately, because of the vaccination attitudes, Covid will be with us for a long time,” Tande said. “By constantly masking and testing before we fly… we can improve safety so we can continue to function as a normal society.”

Clarification: This article has been updated to reflect Dr. Tande on the likely results of repeated surveys.

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