Study aircraft seating
Image Credit: Screengrab

Dubai: Before COVID-19 crippled global travel, the airline industry used to move about 5.5 million passengers per day, or over 2 billion passengers per year.

The recent SARS-CoV-2 pandemic, however, has grounded up to 95 per cent of the world’s commercial planes.

Several studies had been published on the subject of transmission of infectious diseases on board an aircraft.

Experts point to this one fact: Air travel has been associated with the intercontinental spread of new emerging viruses, both via importation of cases and through in-flight transmission.

Let's take a closer look: In 2016, the scientific journal Epidemiology published a review of all the studies available at that time on flu transmission on aircraft.

The study dissected 14 peer-reviewed publications on the subject, describing the follow-up of 2,165 (51%) of 4,252 traceable passengers. We break it down for you, alongside similars studies on the subject.

Here’s what we found so far:

Q: Will I get sick if I sit next or close to someone who is ill?

The risks of infection when sitting next to an “index case” (infected individual), on a passenger plane had been the subject of numerous studies.

A study (published on March 8, 2017) done by Vicki Stover Hertzberg and Howard Weiss on the Annals of Global Health shows there is a ∼6% risk to passengers seated within the 2-rows of infected individual(s).

6%

Risk to passengers seated within the 2-rows of infected individual(s), according to one study.

Moreover, the researchers stated there is ∼2% risk to passengers seated beyond 2-rows from the infectious individual.

Hertzberg, of Emory University, explained that based on their study, the standard contact tracing procedures for airline passengers should include those sitting one row in front of the "index case" (confirmed infected person), one row behind, and two seats on either side.

Q: What is the transmission rate — or the 'secondary attack' rate — on board?

A study published in Epidemiology, meanwhile, states that out of 2,165 (51%) of 4,252 traceable passengers they investigated, they identified 163 “secondary cases”.

This resulted in an overall “secondary attack rate” among traced passengers of 7.5 per cent.

Of these secondary cases, 68 (42 per cent) were seated within two rows of the “index case” (infected passenger).

guidance two rows
Long-standing guidance by public health agencies is that the primary transmission risk associated with air travel for most respiratory infectious diseases is associated with sitting within two rows of an infectious passenger. Other studies, however, dispute this. Image Credit: Annals of Global Health [https://bit.ly/37jlaRV]

Data were available for 239 of 278 (86%) of passengers on the flight, of whom six were considered infectious in-flight and one immune. The "attack rate" (AR) was 10 of 232 (4.3%).

HERE'S ONE IMPORTANT POINT:

The Epidimiology study found no evidence that the "attack rate" for those seated within two rows of an infectious case was different from those who were not.

Q: What's the evidence of an inoperative aircraft cabin ventilation system triggering a high 'attack rate'?

On July 1, 1979, the American Journal of Epidemiology published a study about a classic case of an infection spreading in an aircraft with an inoperative ventilation system.

A jet airliner with 54 persons aboard was delayed on the ground for three hours because of engine failure during a takeoff attempt.

Most passengers stayed on the airplane during the delay. Within 72 hours, 72 per cent of the passengers became ill with symptoms of cough, fever, fatigue, headache, sore throat and myalgia.

One passenger, the apparent index case, was ill on the airplane.  Researchers found that the clinical attack rate among the others varied with the amount of time spent aboard. Virus antigenically similar to A/Texas/1/(H3N2) was isolated from 8 of 31 passengers cultured, and 20 of 22 ill persons tested had serologic evidence of infection with the same virus.

The airplane ventilation system was inoperative during the delay. Researchers said that may have caused the high “attack rate”.

Q: What’s the 2-row rule for infectious disease transmission on aircraft?

It's the rule based on the assumption that the primary transmission risk that goes with air travel for most respiratory infectious diseases is associated with sitting within two rows of an infectious passenger.

This has been a long-standing guidance by public health agencies.

Passengers on board the Khozikode flight
Long-standing guidance by public health agencies is that the primary transmission risk associated with air travel for most respiratory infectious diseases is associated with sitting within two rows of an infectious passenger. The effect of proximity may be one of these factors. Image Credit: Annals of Global Health [https://bit.ly/37jlaRV]

Q: What about droplets from sneezing or coughing? How does flight duration affect the risk?

The Lancet study states that large droplet and airborne mechanisms “probably represent the greatest risk for passengers within the aircraft because of the high density and close proximity of passengers.”

Besides proximity, the spread of contagion to other hosts is dependent on other factors.

These include:

  • Infectiousness of the source
  • Iathogenicity of the microorganism
  • Duration of exposure (or flight)
  • Environmental conditions (ventilation, humidity, temperature)
  • A person's general health and immune status

Researchers pointed out, however, how these factors affect risk of disease transmission within the aircraft cabin is unclear, and must be further investigated.

passenger movement patterns in airline cabins also pose a risk of transmission
Image Credit: Annals of Global Health [https://bit.ly/37jlaRV]

Q: What is the WHO guidance on “contact tracing”?

Current World Health Organisation (WHO) guidance follows the 2-row rule for infectious disease transmission on aircraft. It recommends contact tracing of passengers seated within two rows of a case of influenza during air travel.

There are studies, however, that does not support current 2-row rule regarding the contact tracing of passengers.

Q: What’s the role of on-board ventilation/filtration system?

A (WHO) study stated that the ventilation system on board of aircraft plays a crucial role in keeping infectious diseases at bay.

A functional ventilation system, high-efficiency particulate air filters, and a high air-exchange rate (15 times per hour) is “assumed” to be even more preventive than negative-pressure isolation rooms for multidrug-resistant tuberculosis cases.

Lancet air circulation 01
Image Credit: The Lancet [https://bit.ly/3faQbud]

The researchers cited a study comparing the risk for an upper respiratory infection during air travel in 50 per cent recirculated versus 100 per cent fresh air.

What it found: There was no difference between the two groups.

Commercial aircraft fitted with currently used environmental control system to manage the air flow helps minimise risks for passengers — other than sitting within close proximity to an "index patient".

flight
Image Credit: The Lancet [https://bit.ly/3faQbud]

Q: What is an' index case'?

The index case (also known as “patient zero”) is the first documented patient in a disease epidemic within a population, or the first documented patient included in an epidemiological study. The "term primary case" only applies to infectious diseases that spread from human to human.

It can also refer to the first case of a condition or syndrome (not necessarily contagious) to be described in the medical literature, whether or not the patient is thought to be the first person affected.

In epidemiology, the term is often used by both scientists to refer to the individual known or believed to have been the first infected or source of the resulting outbreak in a population.

Q: What if the aircraft's ventilation system is broken, or is shut down when a flight is delayed?

While in-flight risk for airborne transmission “seems to be minimal”, ground delays without adequate ventilation do pose a significant risk, according to the study.

This is also reflected in a statement by the US Department of Transportation: “If the ventilation system is not operating, passengers should not stay aboard the plane for long time (i.e., more than 30 minutes).”

A potential risk of transmission of the flu via aerosol is also present with a “high attack” rate associated with a ground delay of 3 hours without operational ventilation system.

disinfectant aircraft
DISINFECTING AN AIRCRAFT: Infection at a distance from the index case (infectious person/s on a flight) indicates other factors, such as airflow, movement of passenger/crew members, fomites and contacts between passengers in the departure gate before boarding, or after deplaning, are involved. Image Credit: Annals of Global Health [https://bit.ly/37jlaRV]

Q: So what is the risk of communicable disease being transmitted on board aircraft?

Quick answer: It's difficult to determine.

In the The Lancet study, researchers Alexandra Mangili, MD Dr Mark A Gendreau, MD stated that the risk of disease transmission within the confined space of the aircraft cabin is “difficult to determine”.

This is due to insufficient data, which prohibits analysis, which would allow an idea of the probability of disease transmission for each respective contagion.

Many studies were compromised by reporting bias caused by incomplete passenger manifests, which complicated risk assessment.

The researchers, however, stated: The environmental control system used in commercial aircraft seems to restrict the spread of airborne pathogens, and the perceived risk may be greater than the actual risk.

“Commercial airlines are a suitable environment for the spread of pathogens carried by passengers or crew.”

airline seating rows infectious disease 0001
Image Credit: Annals of Global Health [https://bit.ly/37jlaRV]

Q: So are you saying, then, that cabin air cleaner than city air?

In general, yes. The Lancet study was also confirmed by a study published by the National Institutes of Health (NIH) showed that the concentration of microorganisms in the cabin air is much lower than in shopping malls and the air terminal. 

The study concluded: “The concentration of microorganisms in airline cabin air is much lower than in ordinary city locations. We conclude that the small number of microorganisms found in US airliner cabin environments does not contribute to the risk of disease transmission among passengers.”