AIRLINES have loudly insisted that it’s safe to fly during the coronavirus pandemic, and US travel is surging before the Thanksgiving holiday despite a nationwide spike in virus cases.

Yet top US infectious disease experts say the findings underpinning the carriers’ safety claims aren’t that conclusive.

Concerned about the “misinterpretation” of their findings, researchers on a Defense Department study that has been widely cited by the industry added a cautionary revision. A senior expert in travel-health issues declined to participate in an airline trade group’s press conference, citing what he called their “bad math.” Prompted by the uncertainty, lawmakers on Wednesday called for more in-depth government research.

“The airline industry got a little ahead of itself trying to say the risk is zero,” said David Freedman, a University of Alabama at Birmingham professor emeritus who balked at appearing with an International Air Transport Association event that cited his work.

US airlines, hit with an unprecedented drop in demand since the virus began spreading widely in March, are enjoying their strongest week since then. Even as health officials warn against travel during the Thanksgiving holiday because of a surge in coronavirus disease 2019 (COVID-19) cases, more than 4.9 million people traversed airports between Friday and Tuesday.

The risk of being infected with the novel coronavirus on planes — which have highly effective filters that remove virus from the air and where mask usage is required — is probably fairly low, scientists say.

But the research is far from clear and some recent cases have documented transmission on flights even when passengers wore masks and sat far apart, according to a review of recent cases and interviews with academics and disease specialists.

“I definitely can say it’s premature to say that air travel is very safe,” said Qingyan Chen, an engineering professor at Purdue University in Indiana who’s written extensively on disease transmission on planes.

Airline officials, responding to the historic drop in passengers, have repeatedly defended the protections against infection on flights.

“Flying is safe,” Nicholas Calio, president of Airlines for America, a trade group for large carriers, said at a Nov. 12 briefing. “I will state that categorically.”

A4A declined to add additional comments. It has highlighted the efforts to force passengers to wear masks and to remain apart during boarding and exiting, and to disinfect aircraft. Montreal-based IATA defended its use of Freedman’s data on confirmed in-flight transmissions, saying it never characterized the results as definitive.

A4A has frequently cited a study by the Harvard T.H. Chan School of Public Health, which was funded by the trade group and other aviation industry sources, that concluded the risk of transmission on an airliner was “very low.” But authors cautioned that their projections depended on adherence to mask usage and they also urged airlines to improve ventilation while planes are parked at the gate.

Another study airlines point to was conducted by the Defense Department with the assistance of United Airlines Holdings, Inc. and Boeing Co. It attempted to measure how aerosol virus particles were exhaled by a simulated masked passenger.

United said in promotional materials released Oct. 15 the study “determined the risk is almost non-existent.”

However, after news coverage of the study, the authors added a revision, saying they were “concerned about the potential misinterpretation of the findings.” They also acknowledged they based their results on a person exhaling relatively few virus particles, an amount well below levels documented in some cases.

The airplane filters and mask usage “significantly reduces” exposure to infectious aerosols, they wrote. “However, the current established scientific understanding of SARS-CoV-2 transmission dynamics is not sufficient to calculate definitive SARS-CoV-2 transmission risk from these measurements of aerosol transport.”

United spokeswoman Leslie Scott responded in an email that “Throughout the pandemic, our top priority has been the health and safety of our customers and crew.”

“It’s why we supported military officials, medical experts and aviation engineers in their work demonstrating that aircraft cabins are among the safest of public indoor environments thanks to advanced air filtration systems, required mask-wearing and diligent cleaning protocols,” she added.

Overall, there are few confirmed reports of infections linked to flights. However, because of limited contact tracing in the US and the difficulty of finding transmission cases, it’s hard to say for sure what that means, researchers said.

“I haven’t seen any studies come out and say it’s highly risky,” said Byron Jones, an engineering professor at Kansas State University who has studied airliner cabin-air safety. “But I haven’t seen the study that says it’s definitively safe either.”

The US Centers for Disease Control summarizes the risks from air travel this way on its website: “Most viruses and other germs do not spread easily on flights because of how air circulates and is filtered on airplanes. However, keeping your distance is difficult on crowded flights, and sitting within six feet of others, sometimes for hours, may increase your risk of getting COVID-19.”

Citing what they called the government’s unwillingness to create a plan for handling disease outbreaks in air travel — which is required under an international treaty — and the need for better understanding transmission, US House of Representatives’ leaders on transportation policy asked a watchdog agency to study the issue. 

Representatives Peter DeFazio, the Oregon Democrat who is chairman of the House Transportation and Infrastructure Committee, and Rick Larsen, a Washington Democrat who heads the aviation subcommittee, called on the Government Accountability Office to review research on how the disease is spread in air travel, and the government and industry’s response.

Until a vaccine against the coronavirus is widely distributed, “the US airline industry will depend in large part on a better understanding of how diseases, particularly those that are airborne, spread through air travel and identifying technologies and practices that can help mitigate disease transmission,” the lawmakers said in a letter to the GAO on Wednesday.

While some studies have shown cases in which no one on a flight became infected in spite of the presence of contagious passengers, other data have documented in-flight transmissions.

Purdue’s Chen said he’s been following news reports in China of possible infection between passengers on a Nov. 9 Air China flight from Los Angeles to Tianjin.

Ten people who weren’t connected to each other and resided in different parts of the US tested positive for Covid-19 after arrival. All the passengers had tested negative for the disease before the flight, suggesting at least some of the transmission occurred on the plane, he said.

Such incidents are confounding because they seem to contradict Chen’s own earlier research showing mask usage can dramatically lower risks of infection, he said.

“That’s why I’m having doubts about what’s going on in airplanes,” he said.

Government researchers in Ireland documented as many as 13 cases linked to a single flight last summer, according to a paper published in October. The infections in five of the cases were genetically linked, “strongly suggesting a single point source of infection,” the authors said.

The wide-body jet was largely empty, people were spaced out on the plane and almost everyone whose activity could be documented said they wore masks. Nevertheless, the authors estimated that 10-18% of passengers became infected.

“It is interesting that four of the flight cases were not seated next to any other positive case, had no contact in the transit lounge, wore face masks in-flight and would not be deemed close contacts under current guidance from the European Centre for Disease Prevention and Control,” the authors said. — Bloomberg