COVID Transmission During Air Travel
By Philip R. Fischer, MD, DTM&H
Professor of Pediatrics, Department of Pediatric and Adolescent Medicine, Mayo Clinic, Rochester, MN; Department of Pediatrics, Sheikh Shakhbout Medical City, Abu Dhabi, United Arab Emirates
SYNOPSIS: Despite careful air filtration in flying aircraft, there remains some risk of disease transmission during travel.
SOURCE: Williamson KM, Butler M, Elton B, et al. Transmission of SARS-CoV-2 delta variant from an infected aircrew member on a short-haul domestic flight, Australia 2021. J Travel Med 2022;29:taac144.
During the first six months of 2021, strict COVID-related precautions were enforced in Australia, and COVID was uncommon, with just one case per 100,000 population. However, in June 2021 a flight attendant tested positive for the delta variant shortly after working on an 81-minute domestic flight. Contact tracing was implemented and included all aircrew members (n = 6) and all passengers (n = 139).
During the flight, 88% of business class and 79% of economy seats were used. Two aircrew and eight passengers were infected. They were age 16 to 79 years (median 41 years), most were women, and all were unvaccinated. The genetic identity of the virus was the same in all positive cases.
The index case, a flight attendant, mostly worked in the front (serving the pilots and passengers in the business class section). However, this flight attendant did work briefly near the rear of the plane. This individual did not make contact with passengers outside the plane before or after the flight. Compliant with regulations at the time, the flight attendant wore a face mask, but removed it during public announcements. The flight attendant became symptomatic during the flight and tested positive just after. The other flight attendant working the front of the plane became symptomatic and tested positive for SARS-CoV-2 two days later. Neither flight attendant had been masked while preparing the aircraft together before the passengers boarding.
Eight passengers developed SARS-CoV-2 infection two to seven days after the flight. Three of the seven business class passengers became infected. Three passengers in the middle of the economy section and two in the rear subsequently tested positive for COVID. All subsequently infected passengers admitted to removing their masks during the flight. All subsequently infected passengers had entered or exited via the doors at the front of the aircraft. Comparing passengers who did vs. did not remove masks during the flight (65% removed masks during the flight, at least to eat or drink), mask removal was significantly associated with becoming infected. Seat position (aisle vs. middle vs. window) was not associated with infection.
Williamson et al highlighted the likely transmission of infection from the infected crew member and the association of infection with temporary mask removal during flight. They concluded “wearing masks on board flights remains a sensible measure, especially in light of subsequent, highly transmissible variants where available vaccines may be less effective.”
COMMENTARY
During the early months of the pandemic, Freedman and Wilder-Smith reviewed risks of SARS-CoV-2 transmission during air travel.1 By that time, there had been three flights associated with strong evidence of in-flight viral transmission to multiple passengers, and those flights happened without masked crew or passengers.1 There also had been five eight-hour flights for which passengers were meticulously tested and followed and during which masking was strictly implemented. Despite later awareness there had been SARS-CoV-2-positive passengers on those flights, there was no transmission to other passengers.1 Since then, flight habits changed, and masking was encouraged during air travel. Now, as we emerge from the pandemic and as precautions are relaxed, we still face questions about how to protect ourselves from infection during travel.
Thus, in our current context, the experience from Australia is relevant. Williamson et al remind us that even when COVID transmission is at low levels, risks remain. As in Australia during the 2021 flight and now, there is the possibility of the emergence of new variants of SARS-CoV-2. In-flight transmission remains possible. The risks still come from close contact (within breathing space), and masks remain helpful.
This discussion is as personal for us as it is relevant to our patients and communities. In December 2022, when most passengers and flight attendants enjoyed seeing mask-free smiles, I took two 13-hour flights and carefully used my mask except when I was briefly eating or drinking. By February 2023, when I took two five-hour flights, I, too, was “done” with the pandemic and with masking. I still have never become infected by SARS-CoV-2 (and have been tested frequently enough to think I would have found even asymptomatic infections), but I am not sure my approach was best.
Aircraft air is a 50/50 mix of high-efficiency particulate air (HEPA)-filtered air and fresh external air and is refreshed every two to three minutes.2 Cabin air circulates vertically from ceiling to floor, not along the length of the plane.2 This system is more than 99.99% effective in removing viruses.2 However, there remains a risk of transmission when passing near unmasked faces. I would have been safer during my February flights if I had worn a mask when entering and leaving the planes, since I was in close contact with many other faces and moving through aisles during flights. While vaccination lowers risk, the discomfort and inconvenience of COVID (acutely and with long COVID) remain. Sitting in a plane mostly is safe, as long as no one is coughing or sneezing within a couple seats or rows of me, but walking in planes still carries some risk of becoming infected.
The air travel-related risk of COVID-19 transmission extends beyond the flight time. The authors of a recent report from Canada used modeling and data about flights and population sizes to show the initiation of COVID-19 outbreaks in communities is related to a population’s connectivity by air travel.3 As outbreaks of COVID-19 began (and re-occur as restrictions are eased), the extent of cases initially is linked to population size and the degree of connectivity by air travel.3 In smaller communities, air travel was linked to initial cases, but then local spread became the main means of fueling future outbreaks.3 Further research about the risks of SARS-CoV-2 transmission during travel is needed to best understand what action is most appropriate for diverse groups of people gathered in tight quarters and standing in crowded lines.4 In the meantime, available data will guide personal decisions, and some travelers still should consider wearing masks in some settings.
REFERENCES
1. Freedman DO, Wilder-Smith A. In-flight transmission of SARS-CoV-2: A review of the attack rates and available data on the efficacy of face masks. J Travel Med 2020;27:taaa178.
2. International Air Transport Association. Cabin air and low risk of on board transmission.
3. Hincapie R, Munoz DA, Ortega N, et al. Effect of flight connectivity on the introduction and evolution of the COVID-19 outbreak in Canadian provinces and territories. J Travel Med 2022;29:taac100.
4. Freedman DO. Air travel and SARS-CoV-2: Many remaining knowledge gaps. J Travel Med 2022;29:taac123.
Despite careful air filtration in flying aircraft, there remains some risk of disease transmission during travel.
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