Only 50% of the air on airplanes is recirculated and the recirculated air is filtered through HEPA filters similar to what is used in hospital ICUs.

https://www.who.int/ith/mode_of_travel/tcd_aircraft/en/

"Research has shown that there is very little risk of any communicable disease being transmitted on board an aircraft.

The quality of aircraft cabin air is carefully controlled. Ventilation rates provide a total change of air 20–30 times per hour. Most modern aircraft have recirculation systems, which recycle up to 50% of cabin air. The recirculated air is usually passed through HEPA (high-efficiency particulate air) filters, of the type used in hospital operating theatres and intensive care units, which trap dust particles, bacteria, fungi and viruses."