Studies trace COVID-19 spread to international flights

Three studies published late last week describe in-flight COVID-19 transmission, with one involving a single symptomatic passenger who likely infected at least 12 others during an international flight.

Seat proximity and increased risk

The first study, published in Emerging Infectious Diseases, involved an epidemiologic investigation of all traceable passengers and crew members on a 10-hour Vietnam Airlines flight 54 (VN54) from London to Hanoi, Vietnam, on Mar 1 that resulted in 15 ill people in addition to the index patient.

Researchers interviewed, tested, and quarantined the passengers and crew and traced their close contacts to estimate the likelihood of transmission of SARS-CoV-2, the virus that causes COVID-19, on the flight and identify risk factors for in-flight spread.

Of the 16 crew members and 168 passengers tested, 15 (8.2%) were positive for the novel coronavirus, 12 (75%) of whom had been seated in business class with the symptomatic passenger, for an attack rate of 62% in that section of the 274-seat plane. The other infected travelers had been seated in economy class and may have had contact with the index patient on arrival at immigration or baggage claim.

Of the 12 infected passengers in business class, 8 (67%) developed symptoms after a median of 8.8 days following arrival in Hanoi. None had symptoms on the flight.

Sitting near the ill passenger was strongly tied to increased infection risk. Eleven of the 12 infected passengers sitting one or two seats away (92%) tested positive for COVID-19, versus only 1 (13%) more than two seats away (risk ratio, 7.3).

Of the 1,300 close contacts of crew members and passengers, 5 people (0.4%) tested positive for coronavirus, 3 of whom were household staff members of the index patient, who was the only symptomatic passenger and reported pre-flight contact with her sister, a confirmed COVID-19 patient.

The authors wrote, "The most likely route of transmission during the flight is aerosol or droplet transmission from case 1, particularly for persons seated in business class." They added, "We found no strong evidence supporting alternative transmission scenarios."

The authors noted that thermal temperature screening and self-reporting of symptoms at the airport did not stop the infected person from boarding. As is clear from this investigation, long flights can lead to ideal conditions for "superspreader" events, they added.

Industry guidelines insufficient, authors say

Current international air travel industry guidelines characterize the likelihood of in-flight transmission of SARS-CoV-2 as very low and recommend only the use of face masks—not physical distancing vis a vis blocking middle seats, the researchers said.

"Our findings challenge these recommendations," they wrote. "Transmission on flight VN54 was clustered in business class, where seats are already more widely spaced than in economy class, and infection spread much further than the existing 2-row or 2 meters [6.6 feet] rule recommended for COVID-19 prevention on airplanes and other public transport would have captured." 

The findings reveal the need for more stringent screening and infection-control measures before and during flights, the investigators said, adding that systematic testing and quarantine policies for inbound passengers may also be justified in countries with low levels of community spread, high risk of case importation, and low capacity for contact tracing. Vietnam now mandates testing on arrival and a 14-day quarantine.

"We conclude that the risk for on-board transmission of SARS-CoV-2 during long flights is real and has the potential to cause COVID-19 clusters of substantial size, even in business class–like settings with spacious seating arrangements well beyond the established distance used to define close contact on airplanes," the authors said.

"As long as COVID-19 presents a global pandemic threat in the absence of a good point-of-care test, better on-board infection prevention measures and arrival screening procedures are needed to make flying safe."

Signature virus genome sequence

Another study in the same journal details probable in-flight COVID-19 transmission in two passengers and two crew members on a 15-hour flight from Boston to Hong Kong on Mar 9.

The infected travelers were diagnosed as having coronavirus at local healthcare providers 5 to 11 days after arrival. They included a married couple in business class and two flight attendants, one of whom had served the couple during the flight.

Genomic sequencing showed that the viruses from all four infected travelers were identical, unique, and part of a clade (group of viruses evolving from a single ancestor) not previously seen in Hong Kong, which the authors said strongly suggests that the virus was transmitted during the flight. Similar virus sequences were later identified in Toronto, New York City, and Massachusetts, all of which the couple had visited.

The authors said that the evidence suggests that the married couple were infected in North America and transmitted the virus to the flight attendants.

"SARS-CoV-2 test results have been positive for hundreds of flight attendants and pilots; at least 2 have died," they wrote. "Our results demonstrate that SARS-CoV-2 can be transmitted on airplanes. To prevent transmission of the virus during travel, infection control measures must continue."

Travel with symptoms

A third study, an epidemiologic analysis published in Travel Medicine and Infectious Disease, demonstrated probable in-flight COVID-19 transmission in five passengers. Researchers in Athens, Greece, performed contact tracing on 2,224 passengers and 110 crew members on 18 international flights arriving to or leaving from Greece from Feb 26 to Mar 9.

They identified 21 index COVID-19 cases and 891 close contacts, defined as passengers sitting less than 2 meters away from an infected passenger for at least 15 minutes or crew members who were near them. Of the close contacts, four passengers and one crew member tested positive for COVID-19. Six index patients had symptoms during their flight.

The infected close contacts, who included three members of the same family, had traveled on the same 2-hour flight as two infected travelers who had been part of a pilgrimage to Jerusalem.

"Exposure of the two pilgrim index cases in Jerusalem and subsequent in-flight spread of the disease can be justified considering that pilgrims constitute a high-risk group of travellers for acquisition of respiratory diseases due to their exposure to crowded conditions and mixing with other people from different countries where local transmission of SARS-CoV-2 might had been documented," the authors said.

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