In the days after a man on their flight stopped breathing, fellow passengers wondered if he was infected with coronavirus — and whether they might be at risk. The airline said it didn’t know, and the Centers for Disease Control and Prevention wouldn’t say publicly.

An answer didn’t come until a local coroner released a report a week later confirming that covid-19 was a cause of the 69-year-old man’s death on Dec. 14, along with acute respiratory failure. By Wednesday, three different passengers said they still hadn’t gotten official word from any public health authorities.

“I figured somehow they would get in touch with everybody, send out probably a mass email — at least to everybody who booked the flight — and say, ‘If you were on this flight, you should go get tested,'" said Robert Reeves, 53, who was a few feet away from the man as medical workers performed CPR in the aisle for roughly 45 minutes. The flight from Orlando to Los Angeles diverted to New Orleans so the man could be taken to the hospital, then continued with all passengers on board.

Tony Aldapa, a passenger who helped perform CPR on the man during the flight, said he finally heard from a public health official on Dec. 24 — 10 days after the flight.

Anthony S. Fauci, the top infectious-disease specialist in the U.S., said Dec. 7 that holiday travel plans could create additional coronavirus transmission. (Milken Institute Future of Health Summit)

In an email to The Washington Post, he said he got a voicemail from the Los Angeles County public health department. Aldapa, an intermediate care technician in the emergency department at a Los Angeles hospital, was unable to reach anyone at the number they provided despite multiple attempts. He got an email a few hours later informing him that he had been in close contact with someone who tested positive for covid-19 and ordering him to self-quarantine for 14 days after his contact with the person.

“The most jarring part is thinking about how many people probably didn’t know they’d been exposed and didn’t self quarantine and were out and about because nobody told them,” he said in an email.

The tale of United Flight 591 illustrates the challenges of keeping the novel coronavirus off planes — and informing travelers about possible exposure in a timely manner so they can take their own precautions.

As recent high-profile instances show, having symptoms or even a diagnosis is not enough to keep sick people on the ground. A Hawaii couple who tested positive for the virus was told to isolate in San Francisco; instead, they boarded a plane to Kauai, where they were arrested for reckless endangerment. Authorities in Maryland raced to stop a mother and her son from getting on a plane to Puerto Rico after their test results came back positive. The man on the United flight had been feeling sick before he boarded.

The CDC says in no uncertain terms not to travel while sick or after testing positive for the coronavirus. This year, the agency has added more than 400 people to a “Do Not Board” list for covid-19; those on the list will not be issued a boarding pass for any commercial flight in, to or from the United States.

Travis Nelson, a sergeant with the Maryland State Police and liaison to the state’s emergency management agency, said public health officials can also sign a quarantine and isolation order if they believe someone who has tested positive is about to get on a plane. That’s what happened in the case of the mother and son who were stopped from boarding at Baltimore/Washington International Marshall Airport on Nov. 24.

“It lists the actual law and the penalties,” Nelson said. “The person can be arrested, fined or imprisoned if they violate it.”

To get a wary public flying again, airlines have highlighted the measures they are taking to keep passengers safe, including mask mandates and a health declaration at check-in. But those rely on honest answers, and — as United said was the case last week — passengers don’t always tell the truth.

Medical experts say airlines’ reliance on passengers to self-report any symptoms of covid-19 before a flight is a less-than-perfect way to keep infected passengers off planes.

“Currently everything depends on the honor system,” says David Freedman, a travel and infectious-disease specialist at the University of Alabama at Birmingham.

The man who died submitted a “ready-to-fly” checklist that said he had not been diagnosed with covid-19 and did not have symptoms. But the family later confirmed he had been feeling sick as they prepared to travel; his wife told medical workers on board that he had lost his sense of taste and smell.

Some passengers wondered why United hadn’t done a more careful screening before the flight. Reeves, an actor and amateur filmmaker who lives in Los Angeles, said the Orlando theme parks he and his family had just visited were much more thorough in their health precautions.

“By the time we got to the gate, I was surprised that nobody had even asked us if we had symptoms and nobody had offered to take our temperature,” he said. “If the theme parks can do it — and how many people come in through the gate at a theme park, thousands? — if they can do it, if they can figure out a way to make it work, I think the airlines could and should.”

Some airports and airlines have been requiring preflight temperature checks or offering preflight testing as they try to adapt to the new public health landscape. Testing offered by airlines is generally meant to help passengers meet entry requirements at their destinations or avoid quarantine when they arrive.

Frontier Airlines, which became the first U.S. carrier to require temperature screenings in May, said the number of denied boardings has been “minimal.”

“We believe that given the amount of communication we provide customers on the temperature checks and other health related requirements, plus our flexible change and cancel policies, people are largely not showing up sick,” spokeswoman Jennifer de la Cruz said in an email.

And United and Delta will require passengers traveling from the United Kingdom to the United States to present proof of a negative coronavirus test, the airlines announced on Thursday, following news of a fast-spreading variant in the United Kingdom.

Neither temperature checks nor testing, of course, is perfect. Travelers could be asymptomatic or too recently infected to run a temperature. And someone could be infected but still test negative.

Freedman notes that airline staff are supposed to refuse boarding to people who are displaying symptoms of covid-19, but “obviously they have no medical training, and this is hard to do in the rush of boarding.”

Joshua Barocas, an epidemiologist at the Boston University School of Medicine, says that even if flight staff were vigilant about removing sick passengers, some would get through.

“Individuals can be pre-symptomatic or entirely asymptomatic,” he said. “In both cases those individuals are still infectious, and the symptom screening tools are completely ineffective.”

Barocas also says rising passenger volumes make clear that many people either still need to or are desperate to travel during the pandemic — and therefore could inaccurately self-report their health status.

“In some cases, this is related to business, and in others it is simply because they have taken a calculated risk and the need to see friends and family outweighs the covid risk,” he says. “In all of these cases, it is clear that [their] self-reporting may not be effective.”

Freedman says stringent mask-wearing on flights is an important tool because of this possibility, and that face coverings greatly reduce the risk of transmission if infected passengers are on board. Preflight testing can also help, he says, but it will not keep all infectious people off a plane, “especially when COVID is as rampant as [it is] now.”

It’s impossible to say how many infectious people have taken flights, in part because those travelers may not have known they were sick. The CDC said in September that it had investigated 1,600 cases of people who flew while they were at risk of spreading the virus and identified almost 11,000 people who could have been exposed on those flights. Updated numbers were not available Wednesday, but the agency said earlier in the year that it had not been able to confirm a case of transmission on a plane.

Katherine Estep, a spokeswoman for the industry group Airlines for America, said Wednesday that she was not aware of any documented cases of transmission on U.S. flights.

Globally, documented cases of coronavirus transmission on planes are rare. The International Air Transport Association said in November that there had been “44 confirmed or possible cases” associated with a flight.

Freedman, the infectious-disease specialist, said some countries have adopted practices that might have delivered important news to the United passengers much faster.

“When a flight is discovered to have an infection on board, they post the flight number and the row number of the infected passenger (no names) on a central website with all such flights and leave it there for two weeks,” he said. “So, any person after they fly can check the website for a week or so after the flight to see if there was an issue on their flight. Canada and Australia for example do this; the U.S. doesn’t.”

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