Is it Safe to Fly During the COVID-19 Pandemic?
On the surface, air travel seems like a recipe for COVID-19 disaster — a bunch of strangers packed like sardines into a small space, sharing the same smelly air. Yet, recent news headlines and government policies, suggest that in-flight risks may be smaller than feared.
“Flying can be safer than grocery shopping, Harvard study asserts”. CNN
Is COVID-19 truly not spreading on board airplanes? Or, are industry and government agencies downplaying the risks associated with travel, for economic reasons? Let’s take a look at the evidence.
How likely is it that I will get COVID-19 from flying?
Boarding an airplane clearly increases your “exposure risk” — the risk that you will be exposed to someone with COVID-19 (see Appendix for sample calculation). Indeed, many public health agencies have mounted dashboards that publicly track in-flight exposures (eg. Canadian flight exposures, and British Columbia flight exposures).
Yet, getting exposed to COVID-19 is not the same as getting it. To pass along COVID-19, the infectious person must spew out viral particles, and a critical mass of those particles need to make it into the airways of a fellow passenger. The likelihood that this happens is called the “transmission risk”.
The transmission risk of COVID-19 is tough to pin down, as it varies greatly depending on many factors, including the environment, the individuals involved, and the risk-mitigation practices. As a benchmark, it’s worth noting that two different large contact tracing studies (in India and in South Korea) reported an average 10% transmission risk in households — that is, about 10% of other household members eventually became infected.
How likely is it that an infected airplane passenger will spread it to others?
The emerging consensus is that on-board transmission risk is low. In other words, most COVID-19 cases do not spread to other passengers, but once in a while, we get unlucky. This conclusion rests on two lines of evidence, documented outbreaks and documented lack of transmission, each of which come with important caveats.
Documented in-flight COVID-19 outbreaks
According to a September 2020 literature review, there have been four “mass’ transmissions” of COVID-19 among airlines passengers. At first glance, these stories are pretty scary, such as this infamous flight from London to Hanoi in which one person infected 12 fellow business class passengers as well as main cabin passengers.
A closer look at these mass transmissions is somewhat reassuring, as it suggests that these conditions are unlikely to occur today’s flying environment. Three of four flights had multiple infected people on board (infected passengers = 2,23,27). One of the in-flight cases sat in the same row as five(!) infected people .Three of four flights were low-mask use (pre-mask mandate). The exception was the flight with 27(!) asymptomatic cases. Three of four flights had symptomatic cases. The exception was the flight with 27(!) asymptomatic cases.
Recently, a cluster of cases related to a flight into Ireland sparked new fears, because it was the first mass transmission on a largely mask-compliant flight, and the flight wasn’t even very full. To be frank, it’s hard to know what to make of this new tidbit. Despite extensive contact tracing, the researchers couldn’t determine who brought COVID-19 on board (versus already had it) and how much spread happened before the passengers got on the plane.
In addition, a handful of potential single-case transmissions have been reported (see Sept 2020 review).
How complete is our story of on-board transmissions? As they say, the absence of evidence is not evidence of absence.
To me, it seems unlikely that we are missing many large-scale outbreaks. Such outbreaks will inevitably surface because they are of great interest to public health officials, scientists, and journalists alike.
As for individual transmissions, the answer will vary a lot from place to place. Regions of the world with rigorous contact tracing, and modest COVID-19 rates provide the best opportunity to understand how often COVID-19 cases do NOT spread to other passengers.
Documented lack of transmission
British Columbia (BC) has kept COVID-19 largely in check, in part due to extensive contact tracing and tracking. Over the last three months, BC had 138 exposures on inbound flights, including 63 international and 75 domestic arrivals (BCCDC, Nov 1, 2020). Yet, we have had no reported in-flight transmissions. Even if you assume these databases are imperfect, and miss some cases,this still wouldn’t change the overall low-risk message.
National databases, such as those in Canada, Australia, Hong Kong, which document thousands of in-flight exposures, but little to no outbreaks, offer further encouragement. Hopefully, comprehensive analyses will soon enable more precise estimates of transmission risk.
Given that most known transmissions happened prior to mask mandates, it’s worth noting a handful of reports of no COVID-19 transmission in a subset of high-risk flights that were closely monitored. This 2020 study described six fights carrying a total of 58 infectious cases — yet none of them spread to other passengers. Notably, masks were mandatory in five of these six flights.
What’s going on?
How can we reconcile this evidence of low transmission risk with our understanding of COVID-19 risk? At first glance, airplanes fit all the criteria for high-risk environments — tons of strangers packed liked sardines into a small space, often for a prolonged period. A single whiff of that special airplane aroma attests to the shared air particles.
The answer lies in the multi-layered strategies in place: HEPA filters, airflow, limited interactions between passengers, mandatory masks, and health screening before boarding.
Indeed, the new study that kicked off my deep dive into air travel risks was all about in-flight air quality. They reported that air quality on planes was better (fresher) than many other indoor spaces. Furthermore, when they stimulated a cough on board, they calculated a very low likelihood that that a fellow passengers would inhale enough virus to become infected:
“The test estimated you’d need to be sitting on the same airplane with a COVID-infected passenger for 54 straight hours before you inhaled enough viral particles to become infected. (note, they assumed 1,000 particles needed). US Department of Defense
The Bottom Line
A growing body of evidence suggests that the risk of getting COVID-19 on board an airplane is low — but not zero. There will always be rare, unlucky, exceptions, potentially due to an exceptionally infectious case.
The highest risk flights are those that are long, tightly packed, and departing from places with high rates of COVID-19.
A telling policy change. Here in British Columbia, our leadership recently changed their contact tracing policy such that passengers aboard flights with confirmed COVID-19 cases are no longer directly contacted, even those within the closest five rows. Instead, our provincial and federal governments posts exposures to a public website and post on Twitter.
Should you pack your bags and get some sunshine? Not so fast.
The risks of travel are not just restricted to the airplane. Indeed, public health and infectious disease experts emphasize that the risks associated with travel go far beyond the aircraft. Getting to the airport, getting through security, and simply waiting for your flight can all involve interacting with many people, often in a less controlled way.
Travel also comes with a slew of other considerations. What are the COVID-19 rates in my destination? For those of us who live in Vancouver, just about any trip (typically, south for sunshine) means a greater COVID-19 risk. Will I have to quarantine at my destination? At home? Will my health insurance cover me if I get sick here — or there?
It’s also worth keeping the public health perspective in mind. Even when individual risk is low, public health risk can be non-trivial (as argued in this pre-print study, not yet peer reviewed). As millions of people take to the air, we will eventually see even one-in-a-million events.
All told, while the risk of getting COVID-19 on board is likely low, any trip still warrants requires careful of many factors.
I’m a scientist and mother of three children. My son is beginning grade four, and my twin girls are starting kindergarten.
I completed my PhD in genetics at Stanford and spent the first decade of my career working in cancer research, drug development, and personalized medicine.
My new career chapter is dedicated to empowering others to make well-informed healthy choices, rooted in facts not fears. I’m also passionate about helping people to fall in love with the plants on their plates.
See more of my work, including articles, videos, podcasts, and healthy recipes at: https://FueledbyScience.com
Estimating the risk of being exposed to COVID-19
My very rough estimate of exposure risk when departing from my local airport, YVR, Vancouver International Airport is 1 in 200 or 0.5%. My approach, and assumptions, are as follows:
- Exposure risk = Adjusted COVID-19 case rate x Number of close contacts
- Total active cases per population: 447 cases per 5 million people = about 0.05% or 1 in 2133. Source: BC COVID-19 data Oct 30, 2020: 2344 active cases per 5 million
- Adjusted COVID-19 case rate: Total active COVID-19 cases can overestimate true exposure risk in the community, when many active cases are staying home, either due to symptoms or contact tracing. Given that BC has very strong contact tracing, I dialed down the community rate by a factor of 3 (this assumes people are only out there from days 1–4, but most are home after that from days 5–14). This gets us to an average exposure rate 1 case per 6,000 people in the community. Support for this adjustment comes from the much lower school exposure rates compared to community COVID-19 rates, as discussed in my back-to-school article.
- Number of close contacts: Studies suggest that the main exposure risk group on airplanes comes from 5 rows of people (your row, plus 2 in front and 2 behind). I estimate that the 30 closest people on the plane matter (6 people per row x 5 rows).
How close is close? Most public dashboards (such as Canada’s COVID-19 flight exposure dashboard) report the row the infected person was sitting in, plus two rows on either side.
- Caveats: Using total average for COVID-19 rates assumes that all cases are evenly spread across the region. Yet, there is usually a lot of local variation in COVID-19 rate. All it takes is a few passengers from a local hotspot to dial up the exposure risk. Furthermore, some passengers will come from transfer flights rather than the local region.
Estimating Infection Risk
GETTING EXPOSED IS NOT THE SAME AS GETTING COVID-19!
If forced to put a number on transmission risk, I’d say 1%, or possibly less based on national exposure databases. Combining a 1% transmission risk and a 1% exposure risk would mean a 1 in 10,000 chance of infection on a given flight.