As a doctor, one of the most common questions I’ve gotten throughout the pandemic is whether flying is “safe.”

The question has new urgency, with more contagious variants of Covid-19 spreading around the world, and with the new US policy, effective January 26, that requires all passengers flying into the country to test negative for Covid-19 within three days of their flight. 

The answer to whether flying is safe, as with most questions in this epidemic, is that it’s complicated.

Outbreaks of Covid-19 on flights have been documented multiple times since the start of the pandemic, albeit seemingly more so before masks were widely worn on planes. But one recent outbreak on a long-haul flight from Dubai to New Zealand highlights why flights can be anywhere from safe to unsafe depending on a number of factors. It’s also an example of why viral transmission is not so much a single event, but the result of multiple lapses in safety. 

New genomic data from the flight outbreak this fall sheds light on exactly how complicated tracing the dynamics of transmission in any environment can be — even one as controlled as an international plane ride. 

We still don’t know exactly why this flight turned into a spreading event 

On an 18-hour flight from Dubai to New Zealand this past September, four passengers were infected by one other passenger who boarded the flight unaware that they were infected with Covid-19. Some have pointed to this outbreak as evidence that flying is unsafe, but I think this misses a number of more crucial points.

It’s most important to understand that transmission doesn’t just happen. It actually takes a number of protections falling apart one after the other. This is invoked in the “Swiss cheese model,” in which most single methods of prevention cannot block all transmission, but a layered approach of multiple precautions can stop most spread. 

In this particular outbreak, two of the four passengers who got infected reported wearing masks during the flight. It also happened despite pre-departure testing. The person who brought the virus onto the plane was incorrectly reported as having been tested within 48 hours of the flight, when in fact their negative test was from four days before. The people involved in the outbreak were sitting within four rows of one another, but not everyone in that radius later tested positive for the virus. Additionally, the plane’s power unit was down for 30 minutes during a refueling in Kuala Lumpur, meaning the ventilation system was turned off. 

All of these factors introduce a number of “what ifs” that we can ask about what could have prevented the transmission — and whether improving these steps could make other flights safer. 

For example, what if the index case — the original infected passenger — had been tested within two or three days of the flight? It’s very possible that their infection would have been picked up and they never would have boarded. What if the airline had used rapid antigen testing at the airport, which we know is excellent at detecting people who are very contagious? Of note for this outbreak is that the index case didn’t report any symptoms until two days after the flight, so pre-boarding symptom screening or fever checks also would not have caught them. 

What if the passengers were sitting further from the index case, or if the flight were shorter? We know that distance from index case and duration of contact during travel are both related to a higher virus attack rate.

As of now, only Delta blocks middle seats, and Alaska Airlines does so in the premium section. And in the September outbreak, none of those who caught the virus on the flight were sitting directly next to the index case. Some were two rows ahead, others behind. 

What if the power unit had not been turned off? We know airplanes have great ventilation with HEPA filtration that can block viruses — when the system is running. But there is virtually no way to guarantee a flight won’t have to power off its air systems for unforeseen reasons, such as maintenance issues or de-icing.  

What if the passengers and index cases were all wearing better masks — such as high-filtration KF94, KN95, elastomeric N95, or N95 masks, which can provide better control of virus spread and personal protection? We know that these “hi-fi” masks can function as effectively as vaccines, if not better, in stopping transmission when worn correctly and at times of high risk.

When I tweeted about this outbreak, a number of people quickly chimed in — some saying transmission happened because the vents were off, others saying because passengers didn’t have better masks. Ultimately, we don’t know for sure which of these things it was; more likely, it was a combination of all of them.

What this also demonstrates is that it can actually be somewhat complicated to fully understand how much Covid-19 transmission is happening directly on flights and in other venues. 

We’re probably missing most Covid-19 spread on planes in the US

In the case of this particular flight, New Zealand — which has had a remarkably low incidence of Covid-19 — actually had a mandatory quarantine period during which passengers were in government facilities for 14 days, and were monitored with regular testing. This allowed researchers to isolate the potential transmission points to the flight or airport. But infected passengers reported no close contact with one another in the airport. Genomic studies helped trace the infections as having most likely occurred on the flight itself, given that the viral samples all shared the same lineage. This level of follow-up is rarely happening in the US. 

In the US currently, you can walk off a flight and your quarantine period and subsequent testing are on the honor system, despite being recommended by the CDC. If people don’t strictly quarantine, it quickly becomes much harder to know if transmission happened on the flight or afterward, like on your ride-share from the airport, at your 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 planes become more crowded, community transmission increases, and new, more contagious variants of the virus proliferate, the chance that someone who is actively infected is sitting next to you goes up as well.

One proxy for this risk might be the infection rate among airline staff, and this could be interesting and important to monitor over time. In Canada, infections and exposures on flights are far more readily documented, with a nearly daily list of flights that had infected passengers. Since the beginning of the pandemic, the country’s health officials have identified more than 3,000 flights landing in Canada (domestic and international) on which at least one person had Covid-19. The US would benefit from doing the same, although this needs to be coupled with rapid contact tracing as well.

Does all this mean that flights are dangerous? Does this mean don’t fly? 

I would say that, ultimately, it depends on numerous protections holding up, which at times can and will be out of our control, both for flights and for all other activities that we partake in.

I would recommend against unnecessary travel right now, not only because I am concerned about what happens on the flight (even if many flights are ultimately low-risk) but also because of what happens after the flight. We have no enforcement on quarantining after travel, and many people cannot safely quarantine at home. The more we move around and meet up with others, the more the virus spreads. 

And a single negative test three days beforehand won’t stop that, even if it picks up some infections and prevents some outbreaks on international flights. We should, in fact, be increasing all prevention measures, especially on domestic flights. With new Covid-19 variants, even our airplane travel will require that we do many things right to avoid one especially wrong outcome.

Abraar Karan is a physician 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 Panel for Pandemic Preparedness and Response. The views expressed here are his own.

Covid-19 is now a rapidly growing pandemic, sending countries around the world into states of emergency. It is unprecedented in our lifetimes, what one senior World Health Organization pandemics official called the “Wayne Gretzky of viruses.” The disease has taken over most of mainland Europe, caused major cities in the United States to go into historic lockdowns, and in a matter of weeks has affected every continent except Antarctica — with new countries being affected by the day. 

Accordingly, we need to take measures that have never been done before. These preventative actions, such as social distancing, travel restrictions, and canceling mass gatherings, are likely to slow down transmission and reduce the immediate burden on our health care systems.

Estimates from a very early New York Times model for the United States suggest that aggressive social distancing, widespread testing, and other interventions could reduce Covid-19 infections from a possible peak of 9 million to just 513,000, and total deaths from 982,000 to 51,000 over the span of the next few months. By just staying home (if you can afford to) and avoiding non-essential social activities, hundreds of thousands of lives could be saved. 

While we await the development of a vaccine, these measures are our best bet, and we know that they work from past experiences with pandemic flu. They help to reduce what’s known as the reproductive number (or the average number of cases that one infected case causes), and reduce the peak number of infections at any given time. This is the general idea behind “flattening the curve.” There is a threshold at which the demand on our health care system exceeds the supply, likely leading to many deaths. Social distancing allows us to reduce how much we cross that threshold.

The reactions to the national calls for social distancing have highlighted different extremes. There are people who have already been hiding out in their bunkers with month’s worth of toilet paper and hand sanitizer — and others who were drunk at packed bars for St. Patrick’s Day, as recently as this past weekend. Regardless of which camp you fall into, we will all be affected by the preventative actions our society collectively takes today. 

And yet, the great irony of prevention is that if we do succeed, we may not even know that we did. People who today are being asked to go to great lengths to prevent spread — including missing work, canceling social events including weddings, and staying away from their older loved ones — are doing so at an obvious personal cost, and a less obvious future gain. The problem with prevention is that you don’t always get your name in the rafters; it is hard to appreciate something that did not end up happening. 

While most people could tell you that Covid-19 is “bad,” I doubt most would be able to tell you just how bad it could end up being, at its worst. As a global health doctor who has worked in many low-income countries where the rationing of limited resources — essentially deciding who gets to live — is unfortunately common practice, I know what bad looks like. I can tell you that you don’t want to find out. If we do avoid that worst-case scenario, we may never truly appreciate the fact that we survived.

The same is true with our efforts at preventing the spread of Covid-19. It is likely that if all our coordinated efforts to effectively shut down large parts of our society and economy do end up working to curtail the pandemic, people may later turn around and accuse the public health response to Covid-19 as having been overblown.

I was asked by a medical colleague this past weekend to go out to a bar, despite all of the messaging on social media and by local leadership that has been screaming the opposite. Why such a disconnect? I believe that because we are still at least a few weeks away from the peak of the novel coronavirus outbreak in the United States, most people misperceive their own risk to be what it is in the current moment, rather than what it will inevitably be in a matter of time. 

The issue is that if we wait to truly and strictly implement social distancing, it will be too late. The pandemic will already be a runaway train at that point, and there won’t be much we can do about it except watch our health care providers fight for their lives and the lives of those who we love. 

We can’t let that happen.

We need to take social distancing very seriously, avoiding any unnecessary risks that would put us in close contact with groups of other people. Many friends have asked me about different specific scenarios. A few ground rules will be more helpful. Ask yourself: Am I going to be closer than six feet of someone else at any time? Am I going to be touching objects that are not regularly cleaned or that other people commonly touch? If the answer is yes, you should try to avoid it. 

But social distancing is really better characterized as physical distancing — the social part of it just needs to become more virtual for the time being. Social solidarity has never been more needed. 

We live in a country where our individual rights trump most other things. We need to exercise them wisely, ensuring that we protect ourselves and our society’s most vulnerable by staying home if we can afford to do so, and otherwise taking all necessary precautions if we do need to go out for essential tasks, like getting food or medications.

In the end, the problem with prevention isn’t prevention itself, but our collective failure to appreciate that we still have a chance to change an otherwise bleak future.

Abraar Karan MD, MPH, DTM&H is a physician at Brigham and Women’s Hospital and Harvard Medical School working on population-wide Covid-19 response work. Follow him on Twitter: @abraarkaran.