Health officials in New Zealand, a country that has a strict 14-day quarantine in place for arriving travelers, released a case study on Friday that details the risks of traveling on long-haul flights during the coronavirus pandemic — even if negative coronavirus tests are required before the flight.

The report details a coronavirus outbreak linked through DNA analysis to one passenger on an 18-hour flight from Dubai to New Zealand in September. The traveler, who tested negative for the coronavirus with a polymerase chain reaction (PCR) test within 48 hours of the flight, was contagious but pre-symptomatic onboard the plane, and infected at least four other passengers.

In total there were seven cases linked to the flight, which had 86 passengers onboard.

“By combining information on disease progression, travel dynamics and genomic analysis, we conclude that at least four in-flight transmission events of SARS-CoV-2 likely took place,” the study states. “Four of these six related genome sequences were from Switzerland, the country of origin of the suspected index case.”

New Zealand’s quarantine protocols make the study a unique analysis because all passengers were monitored and retested during their required 14-day quarantine lodging, which is managed by New Zealand authorities. Most flights, doctors have pointed out, have no way of monitoring passengers two weeks after their travel.

“This case speaks to how hard it is to keep infected people off a flight, even if you do PCR testing in a narrow window of time before the flight,” David Freedman, an infectious-disease specialist at the University of Alabama at Birmingham who has reviewed the report, told The Washington Post.

“The original case most likely became infectious after he took the preflight test, but in fact was not symptomatic until 71 hours after the flight,” Freedman said. PCR coronavirus tests are estimated to be about 98 percent effective at detecting the coronavirus, which is why they are required by many countries for entry.

Of the seven infected individuals, five had tested negative within 48 hours before the flight. The authors of the article say the “transmission events occurred despite reported use of masks and gloves in-flight,” and that stringent masking was required by the airline operating the flight.

Freedman points out that the flight length might have impacted masking: “It would have been really hard for people to keep their masks on for the entire 18 hours.”

The evidence contradicts an October Department of Defense study that suggested a contagious person would need to sit next to a passenger for at least 54 hours to infect them, and declared coronavirus transmission risk on planes “low.” It also raises questions about the efficacy of high-efficient air filtration on planes that airlines have credited as keeping passengers safe.

“These seven cases were found to have been seated within four rows of each other during the approximately 18-hour flight,” the study states. “Recent studies have presented conflicting findings of the risks associated with in-flight transmission. We therefore undertook a comprehensive investigation to determine the potential source of infection.”

The aircraft, a Boeing 777, was the same kind used in the Department of Defense study to determine aerosol transmission of the virus was unlikely.

What are the differences between the two types of transmission, how do scientists believe coronavirus spreads, and how does this affect the pandemic? (The Washington Post)

“For both the 777 and 767 airframes, the measured concentrations showed the aircrafts cabin air system was extremely effective in reducing the concentrations of the aerosol tracer particle in passenger breathing zones,” the Department of Defense wrote. “Subsequently, the exposure risk due to aerosols is low.”

Transatlantic flights requiring rapid antigen testing preflight have also become available recently, but rapid tests are only about 70 percent as effective.

On Thursday, the Centers for Disease Control and Prevention recommended against Thanksgiving travel by air and train in the United States, where new coronavirus infections are rising to record levels.

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