Some literature in medicine distinguishes between the “efficacy” of a medicine and the “effectiveness” of a medicine. The efficacy is measured by the medicine’s success in a randomized clinical trial; the “effectiveness” is measured by the medicine’s success in actual clinical practice. Since medicines are usually administered under better conditions, and often to cherry-picked patients, in randomized clinical trails, the “efficacy” of a medicine is almost always higher than the “effectiveness” of the same medicine.
In recent months a study has shown that airline air ventilation technology has very high efficacy in preventing the spread of Covid-19. Yet other studies have shown that in actual practice the “effectiveness” of airline technology has allowed cases of substantial transmission of Covid-19. The article quoted below, plausibly explains that ventilation systems are often turned off, or operate at low levels, when planes are on the ground. Puzzle solved.
(p. A9) The Federal Aviation Administration has few ventilation requirements in cabins, generally deferring to requirements from manufacturers. This has been an issue in the past, when cabins were left without ventilation for long periods with passengers on board. The FAA has issued guidance to airlines recommending passengers be taken off planes if cabin ventilation is shut down for more than 30 minutes, an FAA spokeswoman says.
Leonard Marcus, the director of Harvard’s Aviation Public Health Initiative, says that researchers have found that it’s important to have ventilation running full force on the ground.
“The risk of transmission is increased when people are walking up and down the aisle, when they are putting their luggage in the overhead, when they are breathing on top of one another,” Dr. Marcus says. “So to compensate for that, you have to keep the airflow moving, which is true for all communicable diseases of this nature.”
American says its procedures call for the use of ground air while at the gate and during boarding. Spokeswoman Sarah Jantz says captains have discretion to turn on the APU “if the flow of preconditioned air is not sufficient” or not cooling the aircraft enough. American didn’t change procedures during the pandemic, but did provide additional education to crews “to ensure optimal ventilation,” she says.
. . .
Airline claims about the safety of travel are predicated on functioning ventilation systems. An October  study from Harvard University’s School of Public Health, funded by the airline industry, used mathematical models and found a low risk of coronavirus transmission on airplanes because of a layered approach, including aircraft ventilation and masks.
The study, directed by Dr. Marcus, did recommend extending “in-flight level of ventilation while on the ground.” It didn’t estimate the risk of being in an airplane with even brief periods of little or no ventilation.
For the full commentary, see:
(Note: ellipsis, and bracketed year, added.)
(Note: the online version of the commentary has the date April 7, 2021, and has the title “THE MIDDLE SEAT; The Key to Safe Airflow for Planes Before Takeoff.”)