- Travellers exposed to an infected passenger in an aeroplane in 2020 were tested for the virus.
- It was found that 14 passengers got infected through this index patient.
- Aerosol transmission may have played a key role, highlighting the importance of Covid protocols.
Short domestic flights do not appear to an important contributor to the spread of the SARS-CoV-2 virus, some researchers believe, but according to a new study, this conclusion may not be correct.
Scientists investigated a cluster of Covid infections among travellers and crew members on a two-hour domestic flight in Japan in March 2020. On this flight, there was a passenger (a man in his thirties with symptoms of cough and fever) who did not wear a face mask.
The team then tested the associated risk of infection based on face mask use and the proximity of other passengers to the index patient during this flight. Travellers and crew members were followed up during April 2020 through a validated questionnaire and telephonic interview. Some of the questions involved seat assignment, face mask use during the flight, and symptoms of illness until 34 days after the flight.
According to their findings, published in Influenza and Other Respiratory Viruses, the infected individual infected 14 other passengers on the flight.
Air flight details
The aeroplane was a Boeing 737-800 with 177 economy-class seats, of which 80.2% were occupied. There were 142 passengers, four flight attendants, and two pilots on the flight.
The air recirculation system in the aeroplane was equipped with high-efficiency particulate air (HEPA) filtration. According to the paper, this filtration system had been refurbished in February 2020.
The 146 individuals on board had a median age of 26 years and just over 50% were males. While the researchers identified 14 confirmed Covid-19 cases, they noted that there were an additional six probable cases, excluding potential asymptomatic infections. None of the confirmed cases had had any close contact with a confirmed Covid-infected individual, other than on the flight.
Genome sequencing of the virus was determined in 12 of the 14 confirmed cases and it was established that these were identical to the sequencing of the index patient.
The in-flight transmission likely occurred through infected respiratory droplets and infected micro-aerosols (tiny particles suspended in the air), the researchers noted, but other risk factors for infection included not wearing a face mask, partial face mask use, and being seated within two rows of the index patient.
HEPA filters, physical distancing
Due to HEPA filters, the potential for in-flight transmission of the virus, especially through aerosol transmission, is low, noted the authors.
However, physical distancing policies during flights are still critical. On this specific flight, most of the passengers who got infected were seated further than two rows away from the index case, demonstrating just how far aerosols may travel.
Face masks equally important
In addition to the above, the findings also demonstrate the importance of contact tracing as well as face mask use by passengers and flight attendants during air travel.
The researchers summed up the following as contributing factors to Covid transmission during flights:
- Viral load
- Use of face masks by passengers
- The ventilation system in the cabin
- The location of the infected person's seat relative to the airflow in the cabin
“Wearing face masks during flight travel was associated with a reduced risk of SARS-CoV-2 infection. Now that the spread of variants of SARS-CoV-2 across regions and countries has become a global threat, awareness on the use of face masks, an effective pre-boarding screening system, and a vigorous investigation system are necessary to minimise the risk of in-flight viral transmission,” they wrote.
A critical factor to point out is that, since March 2020, the virus has evolved into more transmissible variants. The Delta variant, for example, is the most contagious variant to date and has become the dominant version of the virus in multiple countries.
While the original virus had an R-value (the number of people to whom one infected individual will pass the virus) of around 2–3 people, one person infected with the Delta variant may infect up to seven individuals, according to a previous Health24 article. This means that, at this stage of the pandemic, there is the potential for a higher number of people to be infected during a flight if there is an index patient infected with Delta.
The researchers wrote: “This cluster occurred in March 2020 before the emergence of several variants of SARS-CoV-2 with confirmed increased transmissibility. In the future, it will be necessary to assess the risk of in-flight transmission of these variants.”
Higher risk during meal service
Recent research also found that while overall risks of in-flight transmission appear to remain relatively low, newer variants may change that equation, the Wall Street Journal reported.
Chances of virus transmission are higher during meal service when passengers remove their masks, it added.
A medical study by researchers at the University of Greenwich in London, for example, found a 59% higher risk of viral transmission during a one-hour meal service on a 12-hour trip, compared with staying fully masked for the duration of the flight.