An accepted manuscript published in the Journal of Travel Medicine by Wang and others on 19th February 2021 looked at this question.
Clearly an issue of concern to anyone travelling is the possibility of in-flight transmission of COVID-19 during long and short haul flights.
The aviation industry has maintained that the probability of contracting the illness is small based on reported cases, modelling and data from aerosol dispersion experiments conducted on board aircrafts.
This study used experimentally derived aerosol dispersion data for a 777 aircraft and a modified version of an equation to estimate the in-flight infection probability for a range of different scenarios involving rate and facemask efficiency.
The mid cabin exhibited the highest infection probability. The calculated maximum individual infection probability without masks for a two-hour flight in this section varied from 4.5% for a mild scenario to 60.2% for a severe scenario, although the corresponding average infection probability varied from 0.1% to 2.5%. For a 12-hour flight, the corresponding maximum individual infection probability varied from 24.1% to 99.6% and the average infection probability varied from 0.8% to 10.8%.
If all passengers wore facemasks throughout the 12-hour flight, the average infection probability was reduced by almost 73% and 32% for high/low efficiency masks. If facemasks are worn by all passengers except during a one-hour meal service, the average infection probability is increased by 59%/8% compared to the situation where the mask is not removed.
The most extreme cases considered involved a 12-hour flight in which the passengers were not wearing masks and in this severe scenario the average probability of infection within the highest risk zone is actually 10.8%. With such a relatively low probability of infection, it may be considered unlikely that an individual would be infected, unless they were in the seat of maximum exposure, in which case they would have a 99.6% probability of being infected.
Furthermore, if all the passengers wore low efficiency masks for the entire 12-hour flight, this reduced the average probability of infection to 6.5% and this is further reduced to 2.8% of all the passengers are wearing a high efficiency mask.
Analysis from this paper suggested that multiple secondary infections can occur on board aircraft, even though the passengers are protected with high ventilation rates and hyper filters although mask wearing mitigates the risk.
On average, the high low efficiency masks reduce the average infection probability by 86%/47% and the maximum infection probability by 73%/32%. Thus, the wearing of masks by all passengers in crews should be considered an essential requirement for all flights and should be worn by all who can wear them at all times.
Furthermore, removing masks even for a short period on a 12-hour flight such as for a one hour meal service increased the average probability of infection by as much as 59% compared to the situation where the mask is worn continuously.
The viral dose received is over eight times greater when no one wears a mask compared to when everyone wears a high efficiency mask.
Furthermore, the viral dose received is almost three times greater when no one wears a mask compared to the case where either the susceptible or in second subject wears a high efficiency mask.
In the severe scenario with a high efficiency mask, removing a mask for one hour results in 2.1 of the 75 passengers seated in the mid-section being infected, which is an increase of 0.8 passengers compared to the case where masks are worn throughout.
It is suggested that when providing a meal service, airlines should keep the mealtime to a minimum and consider alternating the meal service with only half the passengers that are non-neighbouring within a seat row fed at any one time. In this way, the passengers either side of the susceptible passenger currently being fed are wearing masks, this reduces the impact on the maximum infection probability associated with the passenger adjacent to the index passenger.
This will increase the time required for the meal service but will reduce the maximum infection probability. During this staggered meal service, a susceptible passenger seated next to an index passenger will receive a vial dose approximately 62% assuming high efficiency masks of what they would have received during the usual simultaneous meal service.
There is a clear difference in the infection probabilities with the business and first class sections having a much lower infection probability than the mid economy class. The business and first average infection probability is about one-third for the mild and medium scenarios and 42% for the severe scenarios of the mid aft probability.
These differences are probably due to a number of factors such as a lower passenger number resulting in greater seating separations and the business class seat geometry possibly offering greater shielding.
It is not known if the ventilation rate in the business class cabin section is greater than in the economy class section.
Dr Paul Ettlinger
BM, DRCOG, FRCGP, FRIPH, DOccMed