Three studies document in-flight transmission of coronavirus

While in-flight transmission of novel coronavirus among passengers is considered to be low, three studies have found instances where coronavirus transmission had probably taken place in-flight. The spread has probably become possible as mask wearing was not compulsory early in the pandemic. But thermal imaging, which has been in place since the beginning, has been found wanting.

The risk of in-flight transmission of novel coronavirus among passengers is considered to be low. But three studies published recently show that the virus can spread to other passengers. In one study, the researchers found one symptomatic passenger seated in business class had transmitted SARS-CoV-2 virus to at least 15 other passengers during a direct flight from London to Hanoi in early March. While 12 passengers were seated in business class, two other passengers and one flight attendant were in economy class.

Though no genome sequencing study was undertaken to confirm in-flight transmission, the researchers from the National Institute of Hygiene and Epidemiology in Hai Ba Trung District, Hanoi, Vietnam undertook in-depth epidemiologic upstream and downstream investigations, quarantining the passengers and laboratory testing to support their hypothesis of spread during the 10-hour flight. The results were published in the journal Emerging Infectious Diseases.

“Our interviews did not reveal that any of the additional persons with flight-associated cases had been exposed to SARS-CoV-2 before or after the flight during their incubation periods other than having taken the same flight as the index case,” they write. Also, in early March, only 23 COVID-19 cases had been recorded in UK and as of March 1, only 16 cases of COVID-19 had been reported in Vietnam thus reducing the chances of local transmission after arrival; there was no evidence of community transmission in Vietnam in early March.

“The most likely route of transmission during the flight is aerosol or droplet transmission from index case, particularly for persons seated in business class,” they write. In early march, the use of face masks was not mandatory on airplanes or at airports.

Absence of physical distancing in flights

Besides thermal imaging, current guidelines recommends only the use of face masks with no additional measures to increase physical distance on board, such as blocking the middle seats. While masks can cut the risk of transmission, the absence of physical distancing, especially in economy class may increase the risk of virus spread.

“Our findings challenge these recommendations. Transmission on flight was clustered in business class, where seats are already more widely spaced than in economy class, and infection spread much further than the existing two-row or two-meters rule recommended for COVID-19 prevention on airplanes,” they note.

They also note limitations of thermal imaging and self-declaration of symptoms. They also note that it has been “hypothesised that a combination of environmental factors on airplanes (humidity, temperature, air flow) can prolong the presence of SARS-CoV-2 in flight cabins”.

Genome sequencing confirms in-flight spread

Another study, also published in the journal Emerging Infectious Diseases, found probable in-flight transmission of the virus from one or two passengers to two crew members in a 15-hour flight from Boston to Hong Kong on March 9. All the passengers were not tested. The two passengers, who are a married couple, did not have symptoms at the time of flight but developed symptoms a day after landing.

In this study the researchers carried out genome sequencing and found that all the four persons had a sequence that was identical and unique and belonged to a clade not previously known in Hong Kong. “This strongly suggests that the virus can be transmitted during air travel,” the researchers from the University of Hong Kong and National Institute for Medical Research, Mwanza, Tanzania say.

“SARS-CoV-2 test results have been positive for hundreds of flight attendants and pilots; at least 2 have died,” they write. “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.”

A third study published in the journal Travel Medicine and Infectious Disease carried out an epidemiologic analysis to demonstrate probable in-flight transmission of the virus in five passengers. The 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 February 26 to March 9.

The researchers were able to find 21 index cases and 891 close contacts who are passengers sitting less than two meters away from the index case for at least 15 minutes or crew members who were near them. Six index cases had symptoms during the flight, while 14 developed symptoms days after landing; no data was available for one case.

Of the 891 contacts tested, four passengers and one crew member tested positive for the virus. All the four passengers were seated within two seats to the two index cases in a flight from Israel. The two index cases were part of a pilgrimage to Jerusalem.

Published in The Hindu on September 26, 2020