Can airport screenings prevent the spread of COVID-19?
February 9, 2020
The new coronavirus that originated from the Chinese city of Wuhan, also known as COVID-19, continues to spread, and the World Health Organization recently declared it a public health emergency of international concern. As a postdoctoral researcher at the University of Chicago, I study how diseases like this spread and how public health officials can try to contain them.
One of the most common tactics used to try to stop epidemics is screening travelers at airports. Unfortunately, based on research my colleagues and I have conducted on past epidemics and our preliminary findings on the Wuhan coronavirus, traveler screening will miss more than half of people exposed to the virus.
Screening during flu season also means that large numbers of people who are not infected with nCoV are probably being detained as false positives, because the flu has similar symptoms. Despite its obvious flaws, however, screening can have some indirect benefits. To find out why, we have to understand how screening works and how it impacts public awareness during an epidemic.
How do they screen travelers for the 2019 novel coronavirus?
The most common way to screen travelers for infectious diseases is a fever screen where they take someone’s temperature while they’re going through airport security. There is also usually a questionnaire that could ask you, say, in the case of the new coronavirus, if you've been in Wuhan or in close contact with anyone who has the infection. One of the disadvantages right now with the Wuhan coronavirus is that we don't have very specific questions that can help us identify affected people yet.
This kind of screening is different from the travel restrictions recently announced by the U.S. government that deny entry to foreign nationals who visited China in the past two weeks. U.S. citizens returning from mainland China are also subject to up to 14 days in quarantine to make sure they don’t develop symptoms.
While these more stringent measures are designed to prevent the kinds of screening failures we outline in our study, travel and quarantine restrictions will have much bigger ethical, political and economic implications than traveler screening, especially if the epidemic continues inside China for an extended period of time.
How effective is traveler screening for COVID-19?
Unfortunately, we find that these screening methods are not very effective because people have to go through an incubation period when they don't yet show symptoms. You're most likely to travel early in that incubation period while you're still feeling healthy. The incubation period for the coronavirus seems to be about five and a half days on average, but some people are going to have incubation periods that are quite a bit higher than average, meaning they will go longer before they show symptoms.
Furthermore, while an outbreak is still growing, the vast majority of cases are going to have been exposed recently and probably won't have progressed to symptoms yet. That’s another unfortunate reality that makes these screening measures ineffective early in the outbreak when we when we tend to rely on them the most.
What we found is that even in the best-case scenario, screening is probably going to miss over half of people screened. And that's consistent with what we’re seeing in the news. Most of the cases in the U.S. were not detectable when they passed through airport screening. It's been much more common that we detect these cases only after people reach their destination, spend a day or two there and then realize that they're feeling sick and go seek medical care.
How do you mathematically calculate the effectiveness of traveler screening during epidemics?
The model we use is pretty simple. In our simulations, an infected person shows up at the airport and gets screened for fever, and then they are either detected or passed through. Then they fill out a risk questionnaire. Someone can either be aware that they’ve been exposed and self-report that honestly, or the questionnaire is going to miss you. So, we look at the probability of all those different scenarios happening.
We also bring to the model the probability of actually having symptoms that can be detected in that fever screen, given the incubation period. And then in a population, what fraction of everyone who gets screened we expect to be detectable. Essentially, what we see is that a large fraction of the people who would go through screening wouldn’t have any detectable symptoms.
Another complicating factor is that there have been some reports that cases can be mild or asymptomatic. We make assumptions about how long ago an individual was exposed and get a probability for whether they have symptoms yet. We also have to make assumptions about the fraction of any cases that would never show a fever once they did start having any other symptoms.
For the risk questionnaire, we also have to make looser assumptions because we don't know what fraction of people in this outbreak are going to realize that they've had an exposure. From our previous study, we estimate that probably no more than 25% would report honestly if they knew they had been exposed. It's possible that things will be different in this outbreak if people feel more of a moral responsibility to self-report accurately or try to limit onward transmission.
Can screening travelers during epidemics be improved?
Yes and no. The realistic view is that there is just this fundamental biological issue that people are unlikely to show symptoms at the time that they're screened. That puts you in a situation where the most effective way to detect illness is with questionnaires, but we assume that people are unlikely to self-report accurately.
We also know that public health officials often don't have good questions to ask if it's unclear how the disease is spreading at the beginning. So, I think it’s unlikely that screening is ever going to be a super-effective tool.
There are some small things that could improve effectiveness like scanning people for fever twice, in case the first scan is inaccurate. But I think that the improvements from that would be really marginal, just because most people aren't going to have a fever at all, even if they've been exposed. For these reasons I don’t think screening can be thought of as our primary mode of defense against disease spread. There has to be really good on-the-ground follow up instead.
Is screening travelers for COVID-19 worth it?
One way that screening can be beneficial is that it can be used as a way to raise awareness, so that people know that they need to self-isolate and take things seriously if they start developing symptoms.
We live in a culture where people don't take it very seriously when they get sick. People still go into work, they ride public transit, they might go to parties or attend public events when they feel sick, and those are the kinds of behaviors that can have really big consequences when an outbreak is emerging. We call that super-spreading, when one person exposes many people. If screening can raise awareness in a way that reduces the probability that super-spreading happens, then it’s beneficial.
The other reason to screen is that while it's probably never going to prevent the importation of cases, but it could potentially delay the spread of the infection in a new place. That could buy time for medical professionals to learn how to treat the illness more effectively, or it could buy time to develop treatments or vaccines. We’ve seen pretty good implementation of that so far.
There have been a few local chains of transmission in countries other than China, but we haven't really seen big amplification of the epidemic yet outside of China, and it's been almost a month since we realized this was a big problem. We're still not out of the woods here, but I think that the fact that we haven't seen bigger geographic spread so far points to the fact that the people on the ground, the epidemiologists and people working in the hospitals, are doing a pretty good job of helping people contain the spread.
Even though screening isn't effective, it may be possible to stop the global spread of this kind of disease. Any measures that try to break new chains of transmission before they start and reduce the probability of super-spreading make it more likely that we can prevent the number of infected people from multiplying outside of China.
Katelyn Gostic, PhD, is an epidemiologist and postdoctoral researcher in the Department of Ecology & Evolution at UChicago.
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