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Explained: Why risk of virus transmission in an aircraft is seen as low

Studies over the years have sought to quantify the infection risk. Here, we look at two recent ones — neither study, however, looks specifically at the coronavirus pandemic.

Written by Kabir Firaque | New Delhi | Published: May 25, 2020 4:22:06 am
Flights resume, flights open today, flights start today, risk in flights, coronavirus risk in flights, Coronavirus lockdown, Coronavirus domestic flights domestic flights resume india, international flights resume india, flights rules, flights rules and regulation, flights in india, india flight services, india air services, india airlines In general, the risk of a viral infection inside an aircraft is thought to be low, except in cases where healthy fliers are sitting too close to someone carrying an infectious virus. (File Photo)

With domestic flights resuming on Monday, the question being asked is about the risk of fliers being infected with Covid-19.

In general, the risk of a viral infection inside an aircraft is thought to be low, except in cases where healthy fliers are sitting too close to someone carrying an infectious virus.

Studies over the years have sought to quantify the infection risk. Here, we look at two recent ones — neither study, however, looks specifically at the coronavirus pandemic.

One study is from 2018, on droplet-borne virus transmission in aircraft; the other is from this year, focusing on boarding procedures but also looking at onboard transmission.

What is known about such infection?

On infections that spread through droplets, the World Health Organisation (WHO) says: “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.”

About ventilation, it says: “Ventilation provides a total change of air 20-30 times per hour. Most modern aircraft have recirculation systems, which recycle up to 50% of cabin air.”

As flights resume in various countries, aircraft makers Airbus and Boeing have stressed the safety ensured by their ventilation systems, Reuters reported.

From a 2018 study on the transmission of droplet-mediated respiratory diseases during flights, this seating heat map shows the probability of one passenger (triangle) infecting each of the other passengers (squares) and crew members (circle). Probability of infection set at 1.8% per minute of contact. While the infected passenger is shown in seat 14C, the same probabilities apply anywhere except the front and back rows. The study covered transcontinental flights. Source: Hertzberg, Weiss et al, PNAS 2018

What does being “seated in the same area” mean?

For Covid-19, the WHO has prescribed that a distance of 1 metre should be maintained. The 2018 study quantified the infection risk based on proximity to an already infected passenger. The study, by researchers from Emory University and Georgia Tech, was published in the journal PNAS. As a modelling study, it is not a prediction — only a calculation of probabilities based on simulations.

The study chronicled the movements of economy class fliers on 10 transcontinental US flights to draw up a transmission model for respiratory disease spread through droplets. (Movement of passengers and crew can play a key role in virus transmission.)

The figure shows an already infected passenger in seat 14C. The calculated risk of transmission varies, but is very small for most passengers. Only in 11 seats (those nearest to 14C) is the chance of catching the infection high, at 80%-100%. For all other seats, the risk is less than 3%, and reduces with distance. For passengers seated farther away from 14C than 1 m (the WHO stipulation) — which means almost all seats beyond the nearest 11 — the probability of transmission is less than 1%. For a flight attendant who walks the aisle, the risk is between 5% and 20%, according to the study.

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Why specifically seat 14C?

It is just illustrative. “Although we use seat 14C to illustrate this finding, outcomes are similar for an infectious passenger seated in any aisle seat, except in the first or last row, for which no passengers forward or aft, respectively, are infected,” the study said.

The study set the probability of transmission at 1.8% per minute of contact, a conservatively high estimate, it said.

But will it apply to Covid-19?

“Nobody knows these probabilities — for Influenza or COVID!” biomathematician Howard Weiss, one of the study authors, who is now with Pennsylvania State University, told The Indian Express by email.

Flying on Monday or later? Here are the answers to some questions you might have Housekeeping staff clean the walls outside airline counters at Kempegowda International Airport in Bengaluru (AP)

What is the other study about?

Researchers from three institutions in Florida have developed a model to analyse which boarding procedures reduce the risk of disease spread in airplanes. The model of pedestrian dynamics, called CALM, simulates how people maintain a distance from one another. The research was published in the journal PLOS One in March.

“While our focus was on identifying good boarding procedures in general, we looked at sections of the planes, just to get some extra insight,” computer scientist Ashok Srinivasan of the University of Western Florida said by email. Among the findings: “We found that business class carries less risk than economy.”

And does the risk change with a window, middle, or aisle seat?

It depends on air flow. Srinivasan said: “Others have looked at window vs middle vs aisle. If you don’t consider air flow, then aisle seats are more at risk and window less. With air flow considered, things get more complex. In most planes, a window seat would still be safer. But it is possible to have air flow patterns that make window seats riskier.”

The 2018 study assumed omni-directional spread of virions, which Weiss said is a crude assumption. Asked if leaving a seat vacant helps, he said: “Based solely on our study, I cannot justify that. Psychologically, as a passenger, I would certainly prefer it. I could hand-wave that if a passenger is facing forward then the virions coming out sideways have substantially less momentum and will settle more quickly, so the vacant middle seat will provide some protection — but I cannot scientifically justify this.”

What about boarding procedures?

The 2020 study found that it is better to use fewer zones. “If you have many boarding zones, then people tend to cluster together while stowing luggage, which leads to increased contact. The boarding process is faster with more zones, but so is infection spread likelihood,” Srinivasan said.

If flights resume: study estimates outbreak risk View of an empty airport with few visitors at the IGI T3 during the lockdown due to COVID-19 pandemic in New Delhi (Express photo)

But aren’t boarding procedures going to be different now?

Yes, they will be. Indian airports, for example, are doing away with physical check-in. The results of the study by Srinivasan and colleagues, however, are based on the procedures that existed before flight operations ceased around the world.

Also read | Domestic flights resume: Here are the rules for passengers in different states

“Airlines have introduced new boarding procedures in response to COVID-19. We recently completed the basic analysis on these, but have not published them yet,” Srinivasan said. “The above observations still apply. But there are also some interesting insights that would need to wait for a publication!”

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