A version of this article first appeared in Katelyn Jetelina’s newsletter, Your Local Epidemiologist.
On Monday, a Florida judge voided the U.S. mask mandate for public transit, which includes planes, trains, and buses. Several airlines immediately announced they dropped masks. And, in true pandemic fashion, an intense debate about masks has ensued.
There are health equity concerns. There are legal concerns, like setting the precedent that the CDC doesn’t have authority during a public health emergency. And there are epidemiological concerns.
In particular, I’ve noticed dangerous rhetoric around the perceived lack of transmission on planes. This misinformation stemmed from the December 2021 Senate committee hearing when Southwest’s CEO said “99.97 percent of airborne pathogens are captured by filters” so “masks serve no purpose.” While the first claim may be true, the second is not.
Here is a review of the scientific evidence.
Modes of transmission
Like I’ve written before, filtration and ventilation are powerful layers of protection against SARS-CoV-2 and other viruses. Airplanes, in particular, have fantastic systems with an estimated 10-20 air changes per hour. (For context, a hospital has six air changes per hour.) A Department of Defense report found plane ventilation and filtration systems reduced the risk of airborne SARS-CoV-2 exposure by 99 percent. Because of this, transmission occurs less frequently than one might intuitively expect given lots of people in close quarters with shared air. A scientific group reviewed 18 peer-reviewed studies or public health reports of flights published between Jan. 24, 2020, and Sept. 21, 2020, and concluded that “transmission of SARS-CoV-2 can occur in aircrafts but is a relatively rare event.”
But, like any mitigation layer, ventilation/filtration isn’t perfect in stopping transmission. This is because of two things:
You need to get to the airplane, and many spaces, like crowded boarding areas, don’t have great ventilation. Also, filtrations systems are not turned on during the boarding process. One of my favorite aerosol scientists, Jose-Luis Jimenez, documented CO2 levels on his recent international plane trip. The highest CO2 level (the higher the value, the worse the ventilation) was while boarding and taxiing to the runway.
SARS-CoV-2 is spread through aerosols and droplets. Filtration is great for aerosols, which float and suspend in the air for hours. But the air actually has to get filtered first. You can inhale SARS-CoV-2 aerosols before they reach the filter. Also, filtration isn’t effective for larger droplets, which can travel up to 6 feet, but then fall to the ground due to gravity. Masks help with droplets.
Because modes of transmission differ, scientific studies have shown that proximity to the index case (i.e., the person who came on the plane infected and contagious) on a plane impacts risk of infection during a trip. A very extensive study traced all 217 passengers and crew from a 10-hour flight from London to Vietnam in March 2020. At the time, masks were neither mandatory nor widely used. The index case was in business class and symptomatic (fever and cough). The scientists found 16 cases were acquired in-flight (i.e., secondary cases), 12 of which were in business class. This equated to a 75 percent attack rate in business class. Two cases were in economy class, and another case was a staff member.
The importance of proximity is consistent with other viral outbreaks on planes. In a review of 14 studies, researchers found an overall influenza attack rate of 7.5 percent, but 42 percent of the cases were seated within two rows of the index case. A similar finding was documented with SARS on a flight: a 34 percent attack rate within three rows of the index case compared with 11 percent attack rate among persons seated elsewhere.
It’s important to note that there are many examples of secondary cases not in close proximity. In the London to Vietnam study, two cases were not in business class, but instead 15 rows behind in economy, and another was a staff member at the back of the plane. Another study found that 11 people who contracted the virus on the plane were outside the usual parameters (two rows in front and behind).
That’s because people move a lot on planes.
Before the pandemic, a scientific group traveled on 10 intercontinental flights to assess the behaviors and movements of people on planes and the impact on viral transmission. Of the 1,296 passengers observed, 38 percent left their seat once, 13 percent left twice, and 11 percent left more than twice. In total, 84 percent of passengers had a close contact with an individual seated beyond a 1-meter radius from them. People with the most contacts were seated in the aisle.
One would hypothesize, then, that aisle seat passengers have a higher risk of infection. But a SARS-CoV-2 study found the exact opposite. The attack rate was higher for passengers in window seats (seven cases out of 28 passengers) compared to non-window seats (four out of 83). Importantly, the seven window passengers said they never left their seats, too. In another modeling study, scientists found that you definitely don’t want to sit next to an infected person. Beyond that, you don’t want to sit behind them. But you may not be able to decide what seat to take.
Regardless of where you sit on the plane, evidence shows that masks help reduce transmission. Because randomized control trials are not feasible, we’ve had to rely on descriptive and modeling studies to assess the impact of masks on planes.
In a scientific review of studies early in the pandemic, two public health reports extensively assessed transmission rates in the presence of rigid masking. The results affirmed low transmission with masking:
The first flight had 25 index cases but only two secondary cases. One of those two was seated next to a row with five index cases.
On five Emirates Airlines flights with more than 1,500 passengers, no secondary cases were found, despite 58 index cases and food being served onboard (meaning masks were worn most of the time, but not when eating).
A great modeling study was published in 2021 with a few very interesting findings, too:
During a two-hour flight with no masks, the average probability of infection was 2 percent. But if one sat next to an index case, the probability rose to 60 percent.
During a 12-hour flight with no masks, the average probability of infection is 10 percent (or 1 in 10). If one sat next to an index case, the probability rose to 99 percent.
On that 12-hour flight, if everyone wore high efficiency masks the whole time, the probability was reduced by 73 percent. If everyone wore low efficiency masks, the probability was reduced by 32 percent.
If face masks were worn by all passengers except during a one-hour meal service, the probability of infection was decreased by 59 percent (high efficiency masks) or 8 percent (low efficiency masks).
Another modeling study found the impact of masks increased with passenger count on a Boeing 737. With small passenger counts, few passengers were near one another, so mask wearing didn’t make a big impact. But as the plane filled, more passengers were forced closer together, and risk accelerated. So the impact of masks was more apparent.
Transmission on a plane doesn’t just impact those on the plane: Infections will spill over and drive community transmission, too. One study assessed an outbreak on an international flight that landed in Ireland in the summer of 2020. Despite low occupancy on the plane, 13 secondary cases occurred, equating to a 9.8-17.8 percent attack rate. Onward transmission resulted in spread to 59 cases in six of eight health regions in Ireland, which required national oversight.
Planes have great filtration/ventilation systems and vaccines are highly effective, but no mitigation measure is perfect. Wear your mask while traveling, especially with increasing case trends. The layered approach will help reduce your individual-level risk, but perhaps more importantly will help travelers who are high risk and the greater community. To me, wearing a mask is just not that big of an inconvenience for good health.