The key thing as far as transmission is concerned, as I understand it, is that the airflow direction is the first and arguably most important factor.
Aircraft are generally fitted with HEPA filters, but the key characteristic to start with is that aircraft ventilation is negative pressure, with air being sucked out at floor level and drawn in above head height, the same as in hospital high-dependency settings.
In an aircraft, or an environment where temperature is also regulated by the system, there will be an element of recirculation and therefore filtration is appropriate.
However, if air passes in any other direction than directly downwards, HEPA filters are compromised as the air passing in any other direction than vertically down could transmit infections between people long before the filters are reached. Likewise, opening the windows of a moving vehicle might have the principal effect of simply creating an air flow that means that any airborne/aerosol infection at one end of the vehicle could be transmitted swiftly to the other.
These diagrams are from a Chinese research study, showing cases in an air conditioned (non-recirculated) restaurant, published on the US Centre for Disease Control website at
https://wwwnc.cdc.gov/eid/article/26/7/20-0764_article
Abstract
During January 26–February 10, 2020, an outbreak of 2019 novel coronavirus disease in an air-conditioned restaurant in Guangzhou, China, involved 3 family clusters. The airflow direction was consistent with droplet transmission. To prevent the spread of the virus in restaurants, we recommend increasing the distance between tables and improving ventilation.
Extracted conclusion from the report is that:
We conclude that in this outbreak, droplet transmission was prompted by air-conditioned ventilation. The key factor for infection was the direction of the airflow. Of note, patient B3 was afebrile and 1% of the patients in this outbreak were asymptomatic, providing a potential source of outbreaks among the public (
7,8). To prevent spread of COVID-19 in restaurants, we recommend strengthening temperature-monitoring surveillance, increasing the distance between tables, and improving ventilation.