As a doctor, one of the most common questions I get during a pandemic is whether flying is “safe”.
The issue has a new urgency, with more contagious variants of Covid-19 spreading around the world and with the new US policy, in effect on January 26, which requires that all passengers flying into the country have a negative test for Covid- 19 within three days of your flying.
Psaki: “With the worsening of the pandemic and the spread of more contagious variants, this is not the time to lift restrictions on international travel … international travelers to the US must present negative test evidence within 3 days of travel at airlines prior to departure. ” pic.twitter.com/r9mchHE6T9
– Aaron Rupar (@atrupar) January 25, 2021
The answer to whether flying is safe, as with most questions in this epidemic, is that it is complicated.
Flight outbreaks of Covid-19 have been documented several times since the pandemic began, although apparently more before masks were widely used on planes. But a recent outbreak on a long-haul flight from Dubai to New Zealand highlights why flights can be anywhere from safe to unsafe, depending on several factors. It is also an example of why viral transmission is not so much a single event, but the result of several safety lapses.
New genomic data from the flight spurt this fall sheds light on exactly how complicated it can be to track transmission dynamics in any environment – even one as controlled as an international plane trip.
We still don’t know exactly why this flight became a spread event
On an 18-hour flight from Dubai to New Zealand last September, four passengers were infected by another passenger who boarded the flight without knowing they were infected with Covid-19. Some have pointed to this outbreak as evidence that flying is not safe, but I think it omits several more crucial points.
It is more important to understand that transmission does not It just happens. In fact, several protections are undone one after the other. This is invoked in the “Swiss cheese model”, in which most unique prevention methods cannot block the entire transmission, but a layered approach to various precautions can prevent further spread.
In this specific outbreak, two of the four infected passengers reported wearing masks during the flight. This also happened despite testing before departure. The person who brought the virus to the plane was reported incorrectly as having been tested within 48 hours of the flight, when in fact his negative test was four days earlier. The people involved in the outbreak were sitting four rows from each other, but not everyone in that radius later tested positive for the virus. In addition, the plane’s power unit was turned off for 30 minutes during a refueling in Kuala Lumpur, meaning that the ventilation system was shut down.
All of these factors introduce a series of “what if” that we can ask about what could have prevented transmission – and whether improving these steps could make other flights safer.
For example, what if the index case had been tested two or three days after the flight? It is very possible that the infection contracted and they never embarked. What if the airline used a rapid antigen test at the airport, which we know is excellent for detecting very contagious people? Noteworthy for this outbreak is that the index case did not report any symptoms until two days after the flight, therefore, screening for symptoms before boarding or checking for fever would also not have detected them.
What if passengers were sitting further away from the index box or if the flight was shorter? We know that the distance from the index case and the duration of contact during the trip are related to a higher rate of virus attack.
As of now, only Delta blocks intermediate seats, and Alaska Airlines does so in the premium section. And in the September outbreak, none of those who contracted the virus during the flight were sitting next to the folder. Some were two rows ahead, others behind.
What if the power unit had not been turned off? We know that airplanes have great ventilation with HEPA filtration that can block viruses – when the system is working. But there is virtually no way to guarantee that a flight will not have to shut down its air systems for unforeseen reasons, such as maintenance or defrost problems.
What if passengers and index cases were all wearing better masks – like high filtration masks KF94, KN95, elastomeric N95 or N95, which can provide better control of virus spread and personal protection? We know that these “hi-fi” masks can work as effectively as vaccines, if not better, to stop transmission when used correctly and at high risk times.
When I tweeted about this outbreak, several people responded quickly – some saying that the transmission happened because the ventilation openings were off, others saying that passengers had no better masks. Ultimately, we are not sure which of these things was; most likely, it was a combination of all of them.
What this also demonstrates is that it can actually be quite tricky to fully understand the amount of Covid-19 transmission that is happening directly on flights and elsewhere.
We are probably losing most of the Covid-19 spread on planes in the US
In the case of this particular flight, New Zealand – which had a remarkably low incidence of Covid-19 – had a mandatory quarantine period during which passengers stayed in government facilities for 14 days and were monitored with regular tests. This allowed researchers to isolate potential transmission points for the flight or airport. But infected passengers reported no close contact with each other at the airport. Genomic studies helped to track infections as likely to have occurred on the flight itself, since all viral samples shared the same strain. This level of follow-up rarely happens in the United States.
Currently, in the United States, you can abandon a flight and your quarantine period and subsequent tests are in the honor system, despite being recommended by the CDC. If people do not strictly quarantine, it quickly becomes much more difficult to know whether the transmission happened during the flight or afterwards, such as during the trip from the airport, at a relative’s house or during some other activity.
We really don’t have a great understanding of how many infections are happening on flights. And as airplanes become more crowded, transmission by the community increases and new, more contagious variants of the virus proliferate, the chance that someone who is actively infected is sitting next to you also increases.
A proxy for this risk can be the infection rate among airline personnel, and this can be interesting and important to monitor over time. In Canada, infections and exposures on flights are documented much more easily, with an almost daily list of flights with infected passengers. Since the start of the pandemic, the country’s health authorities have identified more than 3,000 flights landing in Canada (domestic and international) in which at least one person had Covid-19. The United States would benefit if they did the same, although this also needs to be coupled with quick contact tracking.
Does all this mean that flights are dangerous? Does that mean not flying?
I would say that, ultimately, it depends on the support of innumerable protections, which sometimes can and will be beyond our control, both for flights and for all other activities in which we participate.
I would not recommend unnecessary travel now, not only because I am concerned with what happens on the flight (even though many flights are ultimately low risk), but also because of what happens after the flight. We have no supervision over the quarantine after the trip, and many people cannot safely quarantine it at home. The more we move and meet other people, the more the virus spreads.
And a single negative test three days before will not prevent this, even if it catches some infections and prevents some outbreaks on international flights. We should, in fact, increase all preventive measures, especially on domestic flights. With the new Covid-19 variants, even our air travel will require us to do many things right to avoid an especially wrong outcome.
Abraar Karan she is a doctor at Brigham and Women’s Hospital and Harvard Medical School. He previously worked on the Massachusetts State Covid-19 response and is a consultant to the Independent Pandemic Preparedness and Response Panel. The opinions expressed here are private.