Two early release studies published in the US Centers for Disease Control and Prevention’s (CDC) journal, Emerging Infectious Diseases, have analysed the risk of getting infected with Covid-19 on a flight. Both studies, however, have analysed flights that operated in early March, before the WHO declared the outbreak a pandemic and before airlines established protocols such as requiring passengers to wear masks.
Now, the International Air Travel Association (IATA) recommends that passengers wear a mask for the entire duration of their journey – from the time they enter the airport to when they exit their destination airport.
It maintains that the risk of contracting the infection is lower on an airplane than at a shopping centre or an office because the cabin air on an airplane is changed frequently.
The first study
The first study assessed the role of in-flight transmission of SARS-CoV-2 on a 10-hour commercial flight from London to Hanoi in early March. The authors of this study conclude that, “In-flight transmission that probably originated from 1 symptomatic passenger caused a large cluster of cases during a long flight.”
This flight had 16 crew members and 201 passengers on board. The 274 seats on the aircraft were divided into 28 business class seats, 25 seats in the premium economy class and 211 seats in the economy class. Out of the 201 passengers on this flight, 21 occupied business class (75 per cent occupancy), 35 occupied premium economy (100 per cent occupancy) and 145 occupied the economy class (67 per cent occupancy).
In this study, the authors note a 27-year-old businesswoman from Vietnam as a probable index case. The index patient was seated in the business class and at the time she took the flight, was already experiencing a sore throat and cough. Overall, 15 people on board were eventually confirmed to be Covid-19 positive. Of these 15 people, 12 had travelled in business class, two in economy class and the remaining person was a flight attendant also in the economy class. Eleven of these passengers were seated less than 2 metres away from the index patient, which the authors have taken to be a distance of at most 2 seats away. Further, one person whose seat was more than two seats away from the index patient, also tested positive.
Significantly, the authors note that there is no strong evidence supporting potential transmission of the infection either before or after the flight. “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 note.
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The second study
In the second study, researchers examined public records for more than 1,000 persons with laboratory-confirmed Covid-19 in Hong Kong from January 23 to June 13. The authors identified a cluster of four persons with Covid-19 associated with a commercial flight that departed from Boston on March 9 and arrived in Hong Kong on March 10. The airplane flew for 15 hours and carried a maximum of 294 passengers.
These four individuals consisted of two cabin members and two passengers. Of these, patients A and B were a married couple and patient A sat on the window seat in business class, while his wife sat directly opposite to him, also in a window seat. Patients C and D were both flight attendants. Patient C served patients A and B.
The authors conclude that “the most likely sequence of events is that one or both of passengers A and B contracted SARS-CoV-2 in North America and transmitted the virus to flight attendants C and D during the flight.” Further, they conclude that patient D could have acquired the infection from patient C, “but because their test results were positive within 1 incubation period, it is more likely that patient D was infected by patient A or B.”
The authors emphasise that their results “strongly suggest” in-flight transmission.
What do we know about in-flight transmission of infection?
In general, the risk of transmission of a viral infection inside an airplane is considered to be low, except in cases where healthy fliers are sitting too close to someone carrying an infectious virus.
On the spread of infection through droplets, the WHO has said, “Transmission of infection may occur between passengers who are seated in the same area of an aircraft, usually as a result of the infected individual coughing or sneezing or by touch… Highly contagious conditions, such as influenza, are more likely to be spread to other passengers in situations where the aircraft ventilation system is not operating.”
What is the reason for low rate of transmission on airplanes?
The IATA maintains that while the reasons for the low rates of in-flight transmission are unknown, some of the possible causes could be the lack of face-to-face contact, the physical barriers provided by seat backs, and characteristics of the airflow.
The WHO has said that ventilation on airplanes provides a total change of air 20-30 times per hour, and most modern aircraft have recirculation systems that can recycle up to 50 per cent of the cabin air.
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