Monday, May 25, 2020

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

New Delhi: 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.
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.

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.

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.

“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.
25/05/05 Kabir Firaque/Indian Express
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