After an 18-hour flight from Dubai to New Zealand, seven passengers tested positive for Covid-19 while under managed isolation and quarantine. Analysis of the virus’ genetic code—along with details about the passengers’ symptoms and behavior throughout their trip—indicates that one passenger infected at least four others while aboard the plane, according to a study published in Emerging Infectious Diseases.
Air travel has been difficult to study because different airlines and countries have different safety policies, and all policies rely on passengers’ willingness to follow the rules. The cluster of cases shows how precautionary measures, like obtaining a negative Covid-19 test result before a flight, aren’t enough to prevent transmission of the virus if other safety measures like mask-wearing aren’t strictly followed, Harvard Medical School physician Abraar Karan writes for Vox. It also shows how the managed isolation and quarantine, or MIQ, system successfully prevented the travelers from sparking new community spread of the disease, the researchers write.
The study found that two people, travelling together, got on the flight in Dubai who had gotten a test for Covid-19 four days earlier. Their tests came back negative before the flight, but one of them began showing symptoms two days after arriving in New Zealand, and another test on the third day returned a positive result for both individuals. The pair said that they wore masks and gloves while on the plane, but took their masks off when they were seated, sleeping or eating, Marc Daalder reports for the New Zealand-based Newsroom.
On such a long flight, people need to eat and drink, but each instance makes it more likely that the virus will spread.
“It is surprising and not surprising, on an 18-hour flight, that an outbreak would occur,” says Karan to the New York Times’ Benedict Carey. “It’s more than likely that more than just those two people took off their mask at some point.”
The flight also stopped to refuel in Kuala Lumpur, Malaysia, so the air filtration system was turned off for 30 minutes.
During the 14-day quarantine period in New Zealand, five other travelers from the same flight tested positive for Covid-19. All of them sat within two rows in front of or behind the two people who tested positive, with symptoms, on the third day. (Another person tested positive on the third day but didn’t show symptoms.)
As the passengers’ positive test results came back, scientists gathered the virus’ genetic information in order to trace back where they had gotten infected.
"We found that a bunch of genomes from that analysis, that was a routine surveillance analysis, were linked," says University of Otago evolutionary biologist Jemma Geoghegan, also a member of New Zealand’s Covid-19 sequencing team, to Newsroom. “They were from quite geographically separated countries but only got on the same flight from Dubai, so their connecting flight was the only time that they had actually been in contact with each other."
Karan writes for Vox that the cluster of cases is a reminder of the “Swiss cheese model” of risk management, where each precaution has a few holes, but when they’re stacked together, risk can be greatly reduced. In this case, the researchers conclude that pre-flight testing is not enough on its own to stop Covid-19 cases from arriving unannounced.
“The idea that people can test negative and then get on a plane and you're all fine, that's clearly nonsense. People could be incubating, they can get infected in the few days before they fly,” says microbiologist Siouxsie Wiles to the Newsroom. “That just again is really good evidence that the pre-flight testing, while it would stop people who are infectious from getting on the plane, won't catch everyone."
Wiles tells the Guardian’s Elle Hunt that New Zealand’s managed isolation and quarantine system is also not foolproof—at least one woman may have caught the virus while at the government-run MIQ center, and tested positive after leaving, so ongoing surveillance of community spread is necessary. But by treating all international travelers as possible carriers of the coronavirus, the country can catch cases that pre-flight testing misses.
While the United States has just instituted an order requiring international travelers to present negative test results before arriving, post-flight quarantining and additional Covid-19 tests are not enforced.
“We really don’t have a great understanding of how many infections are happening on flights,” Karan writes for Vox. “And as planes become more crowded, community transmission increases, and new, more contagious variants of the virus proliferate, the chance that someone who is actively infected is sitting next to you goes up as well.”