Coronavirus airport screening "not effective"

Why questionnaires and thermometers at airports miss most cases of travellers with diseases...
21 April 2020

Interview with 

Katie Gostic, University of Chicago


An aircraft in flight


While air travel has spread this virus around the globe, there have been differing opinions amongst governments and scientists about the merits of closing down airline routes. Some countries have stopped flights altogether but many planes are still flying, albeit usually with screening measures at airports. But as Chris Smith has been hearing from Katie Gostic at the University of Chicago, this screening may not actually be effective...

Katie - The sort of traveler screening historically has not been particularly effective for other pathogens, but we were trying to get a better estimate for how it might play out for this coronavirus.

Chris  - What sorts of screening are and were people doing?

Katie - The typical screening approach involves a symptom screen, which basically means that your temperature is taken and if you have obvious respiratory difficulties or a cough that would be noted by whoever's doing the screening. And then there's usually also a questionnaire that asks you things about your risk factors. So for example, have you been in an area where we know that there's a coronavirus outbreak.

Chris - And when you look into this, actually what was your approach?

Katie - The first thing we did was we asked, what's the probability that any single individual passing through screening would be caught? To do that you basically need to come up with a probability that an infected person is detected in the symptom screen and then another probability that they're detected in the questionnaire-based screening. We can come up with those probabilities based on what we know about the biology of the virus. We basically estimate that people are unlikely to realise they have been exposed to coronavirus, based on the fact that a lot of cases were showing up early who weren't able to report a clear source of exposure. And then in terms of the probability of having started to show symptoms, we can estimate that based on how long the incubation period of the virus is known to be.

Chris - Do you then take, what, real world travel data? And ask, “with these assumptions, how many would we find?”

Katie - We can test how well it's working by comparing it to real world data. Basically to do that, you know at what time different countries had implemented airport screening, and once someone passes through screening and later ends up in the hospital, then we know that screening has failed to detect that person.

Chris - Right. And on the basis of your model and your assumptions, how effective is this screening?

Katie - So we estimate that even in the best case scenario, that screening is probably missing 50% or more of infected travelers with coronavirus. The vast majority of people who would feel well enough to travel probably don't show symptoms at the time that they pass through screening. And we know that the incubation period for this pathogen is pretty long. And we know that there are some people who are completely asymptomatic. And so basically those people who either don't yet show symptoms or might never show clear symptoms are just undetectable.

Chris - Gosh, 50%. That's enormous.

Katie - Yes. We think that this is one of the big reasons that the virus was able to spread so easily.

Chris - We are where we are now: we've got a quarter of the world's population, in late March, currently experiencing some kind of lockdown. What can we take away from what you've found here, that obviously it may be a bit late for this situation, but so that we don't get SARS-Coronavirus mark three in the future?

Katie - I think that's a difficult question. One obvious extreme end of the potential solutions would just be to shut down air travel networks as soon as we detect the next emerging infectious disease. That obviously comes with serious economic implications, but on the other hand, what we've seen here is that it's really difficult to screen for emerging infectious diseases at airports and that air travel is a really important driver of global spread. And so usually the way that public health agencies respond to these scenarios is that as soon as the case is detected, public health professionals try to round up anyone that the first imported case might've been in contact with and infected, before those people can start new chains of transmission. People in my field often talk about new epidemics as being like fires, and these first imported cases like throwing sparks on the ground. We often can't prevent every spark from coming in, but we can do our best to prevent a big fire from starting.


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