While this article from medRxiv has not been peer-reviewed, the study found that the COVID-19 mortality risk to uninfected passengers is less than one in 1 million.
The article states that recent study shows that when all coach seats are full on a US jet aircraft, the risk of contracting COVID19 from a fellow passenger is 1 in 7,000. But even with the middle seat empty policy, the risk falls to 1 in 14,000. Risks would be lower in lesser full flight. Their estimates imply that while the risk of filling up middle seat with another passenger is higher than those associated with plane crashes, the probability is still LESSER than one in 1 million.
Less than one in 1 million probability of mortality risks from the virus while traveling on a jet plane even with every seat occupied, sounds reasonable considering that the trajectory of water droplet from a coughing person is forward and toward the back of the seat in front. However, even with the middle seat open, the space between the people sitting in that row is maybe 2 feet. But the space between the person sitting either in the front or the back of you is still much closer than the recommended 6 feet distance from the Center of Disease Control.
The article was authored by Arnold Barnett(firstname.lastname@example.org) MIT Sloan School of Management.