Leaving middle seats on aircraft open reduces passengers' exposure to Covid-19 by 23% to 57%, according to a CDC study released Wednesday.
The findings come as Delta Air Lines, the final U.S. airline blocking middle seats, prepares to end that practice at the end of the month.
However, authors stress that the study charts Covid-19 exposure risk, not transmission risk, and it does not consider the impact of mask-wearing.
For the study, researchers at the CDC and Kansas State University tested single-aisle aircraft using a five-row section of a Boeing 737 fuselage. They tested widebodies using an 11-row section of a Boeing 767 cabin mock-up. The researchers used spray bottles to disperse a virus called bacteriophage MS2 as a surrogate for Covid-19.
In the first metric, authors looked at the impact of removing a middle-seat passenger from a single row. Doing so reduces the risk of Covid-19 exposure for the remaining flyers in that row by 23%, they found.
In the second scenario, the researchers considered the impact of removing middle-seat passengers across an otherwise full 120-seat plane in which one, two or three passengers are infected. A 35% to 39.4% reduction in exposure was found compared with when the plane is full.
In the final research approach, the authors considered a theoretical cluster of nine infected passengers among three rows that had a total of 18 seats. When a third of those passengers were removed, including three who were infected, exposure to the virus reduced by 57%.
The authors acknowledged that the study has limitations, including not considering the impact of masks, which should reduce exposure risk.
But the limitations cut both ways. The study only assessed virus aerosols, not larger droplets. Since exposures decrease more rapidly with distance for droplets, leaving middle seats open would have an even larger protective effect for droplet exposures than they do for aerosols.
Current CDC guidelines recommend against travel for people who have not been vaccinated.
"Physical distancing of airplane passengers, including through policies such as middle-seat vacancy, could provide additional reductions in [Covid-19] exposure risk," the authors wrote.
Along with Delta, Alaska, Southwest, JetBlue, Hawaiian and Frontier blocked at least some seats early in the pandemic.