How likely is it that you’ll catch Covid-19 during a flight? An estimate for the time frame of June 2020 to February 2021 is provided by a study conducted by MIT scholars. The study provides a strategy that might be modified as the Covid-19 epidemic spreads, even though conditions at that time were different.
The study estimates that from mid-2020 to early 2021, the probability of contracting Covid-19 on an airplane exceeded 1 in 1,000 on a fully packed two-hour flight at the peak of the early pandemic, approximately December 2020 and January 2021. It dropped to approximately 1 in 6,000 on a half-full two-hour flight when the pandemic was at its lowest point in the summer of 2020. The overall risk of transmission from June 2020 through February 2021 was approximately 1 in 2,000, with a mean of 1 in 1,400 and a median of 1 in 2,250.
To be clear, current conditions differ from the study scenario. Masks are no longer mandatory for U.S. domestic passengers; in the study period, airlines used to leave middle seats open, which they no longer do; new variants of Covid-19 are more contagious than the virus during the study period. Although these factors may increase the current risk, most people have received vaccinations against Covid-19 since February 2021, which could reduce the present risk. However, the precise impact of those vaccinations against the new variants is uncertain.
Still, the study provides a general estimate of air travel safety concerning Covid-19 transmission and a methodology that can be applied to future studies. Some U.S. carriers claimed at the time that on-board transmission was “virtually nonexistent” and “almost nonexistent,” but as the research shows, there was a discernible risk. On the other hand, passengers were also not exactly facing the odds of contracting the virus during the flight.
“The goal is to state the facts,” says Arnold Barnett, a professor of management at MIT and an expert in aviation risk, who co-authored a recent paper detailing the study’s findings. “Some people might say, ‘Oh, that doesn’t sound like much.’ But if we at least tell people what the risk is, they can judge it.”
As Barnett also notes, a round-trip flight with a change of aircraft and two two-hour segments in each direction counts as four flights in this accounting, so a 1-in-1,000 chance per flight would lead to an approximate 1-in-250 chance for that trip as a whole.
In total, considering 204 million U.S. domestic airline passengers between June 2020 and February 2021, the researchers estimate that about 100,000 cases of Covid-19 were transmitted on flights during that time.
The article, “Covid-19 infection risk on U.S. domestic airlines,” appears in advance online this month in the Health Care Management Science journal. The authors are Barnett, George Eastman, Professor of Management Science at MIT Sloan School of Management, and Keith Fleming, a student in MIT Sloan’s master’s program in business analytics.
Barnett is a veteran aviation safety expert who has analyzed, among other topics, the long-term reduction of aviation accidents in recent decades. The current study of Covid-19 virus transmission was prompted by an airline policy change early in the pandemic: Delta Air Lines began leaving middle seats open on domestic flights to de-densify its planes, a practice that some other airlines followed for a time. (Delta and all other airlines no longer use this policy).
To conduct the study, Barnett and Fleming amalgamated public health statistics on the prevalence of Covid-19, data from peer-reviewed studies on the mechanisms of Covid-19 transmission, data on the spread of viruses on airlines in general and the spread of Covid-19 on international airlines, and some available industry data on seat occupancy rates on U.S. domestic flights. They then calculated transmission risks on U.S. domestic airlines using extensive modeling.
The researchers used a two-hour flight for their estimates because that is the average duration of a domestic flight in the U.S. For aircraft configuration, the researchers used a Boeing 737 and an Airbus A320, U.S. business jets with a single aisle, three seats on each side, and a typical capacity of about 175 passengers. Most aircraft have high-performance HEPA air purification systems, which help reduce the risk of airborne disease transmission.
Taking the prevalence of Covid-19 in the United States as a starting point and integrating airborne transmission data, Barnett and Fleming modeled what would likely occur on flights with various passengers. The modeling includes several adjustments to make the passenger profile as realistic as possible. For example, airline passengers are slightly more affluent than the U.S. population, and Covid-19 has affected more affluent populations slightly less than other social groups. These aspects are quantified in the study, among other factors.
Ultimately, Barnett and Fleming found a marked decrease in the risk of transmission when planes have fewer people on them, either because of lack of demand or because airlines vacate middle seats. While leaving middle seats open does not eliminate all proximity to all other passengers, it reduces the degree of proximity to others and thus decreases the overall risk of transmission.
“The [medical] literature suggests that proximity is important,” Barnett says.
As Barnett points out, pandemic circumstances and airline policies are still evolving, so his estimates for the 2020-2021 period in the study may not translate accurately to the summer of 2022. Despite the availability of vaccines, he believes that less masking, busier flights, and the easy transmissibility of current variants mean that risks may have increased.