The versions of the coronavirus that all seven carried were virtually identical genetically — strongly suggesting that one person among them initiated the outbreak. That person, whom the report calls Passenger A, had in fact tested negative four or five days before boarding, the researchers found.
“Four or five days is a long time,” Dr. Karan said. “You should be asking for results of rapid tests done hours before the flight, ideally.”
Even restrictive “Covid-free” flights, international bookings that require a negative result to board, give people a day or two before departure to get a test.
The findings are not definitive, cautioned the authors, led by Dr. Tara Swadi, an adviser with New Zealand’s Health Ministry. But results “underscore the value of considering all international passengers arriving in New Zealand as being potentially infected, even if pre-departure testing was undertaken, social distancing and spacing were followed, and personal protective equipment was used in-flight,” the researchers concluded.
Previous studies of infection risk during air travel did not clearly quantify the risk, and onboard air filtration systems are thought to reduce the infection risk among passengers even when a flight includes one or more infected people. But at least two recent reports strongly suggest that in-flight outbreaks are a risk: one of a flight from Boston to Hong Kong in March; the other of a flight from London to Hanoi, Vietnam, also in March.
On the Hong Kong flight, the analysis suggested that two passengers who boarded in Boston infected two flight attendants. On the Hanoi flight, researchers found that 12 of 16 people who later tested positive were sitting in business class, and that proximity to the infectious person strongly predicted infection risk.
Airline policies vary widely, depending on the flight and the carrier. During the first months of the pandemic, most U.S. airlines had a policy of blocking off seats, or allowing passengers to reschedule if a flight was near 70 percent full. But by the holidays those policies were largely phased out, said Scott Mayerowitz, executive editor at The Points Guy, a website that covers the industry.