The CDC says airplanes should leave middle seats empty to reduce COVID-19 spread - after every major US airline decided it was unnecessary

• A CDC study estimated that keeping middle seats empty may reduce "viable virus" exposure by up to 57%.
• In the modeling study, the mannequins didn't wear any masks.
• It's a reminder that masks plus distance is best, especially if passengers remove their masks.

This summer, every major US airline will have middle seats open to purchase on planes.

But a new Centers for Disease Control and Prevention study suggests that could be a great way to invite COVID-19 to spread aboard flights, especially when passengers might lower their masks to eat or drink.

The CDC report, out Wednesday, estimated how far coronavirus aerosols (airborne particles that can linger) could spread on a plane. To do this, researchers modeled how a similar virus spread among mannequins in a lab when they weren't wearing masks.

The study found that leaving middle seats empty to space out passengers and their germs could help reduce viral exposure in flight when people take off their masks or if their masks have gaps in them.

For instance, the study found that sitting two seats from an infectious person could reduce a passenger's risk of exposure to "viable virus" particles by 23% compared with their risk sitting right next to an infectious person with their mask off. That risk could be reduced by as much as 57% in a three-row section with vacant middle seats.

While masks are now required apparel on major airlines and very few cases of COVID-19 spreading aboard planes have been documented during the pandemic, masks aren't perfect at trapping viral aerosols in flight.

A recent case study from New Zealand suggested that even masked airline passengers who have COVID-19 could emit aerosols capable of infecting passengers around them.

The new CDC study, though an imperfect approximation of how people behave on planes during the pandemic, is a good reminder that layers of protection - masks, distance, and frequent handwashing - are much better at preventing the spread of the virus than a single protocol.