Air enters the airplane cabin from overhead inlets and flows downward towards floor level outlets.
Air enters and leaves the cabin at the same seat row or nearby rows.
There is relatively little airflow forward and backward between seating rows, making it less likely to spread respiratory particles between rows.
The airflow in current jetliners is much faster than normally occurs in indoor buildings.
Half of it is fresh air from the outside, and the other half is recycled through HEPA filters-same type used in the operating rooms.
Therefore any remaining risk of the spread of Covid-19 relates to contact with other passages who might be infectious.
Blocking seats provides a partial physical barrier, but still there is very little face to face contact as most flyers remain relatively still.
The number of suspected or confirmed cases of in-flight Covid-19 transmissions in between passengers around the world is approximately 42 in total (as of 11/3/20.
To lower risk: temperature testing, asking about symptoms of fever loss of sense of smell, chills, cough, shortness of breath, enhanced cleaning and disinfection, contactless boarding luggage processing, use of physical barriers and sanitation in airports, physical distancing in airports and during boarding, use of face coverings or masks, separation between passages when feasible, adjustment of food and beverage service to reduce contact, control of access to aisles and bathrooms to minimize contact, limiting exposure of crewmembers to infection, facilitation of contact tracing in the event that a passenger develops an infection.