Screening for disease spread
When disease outbreaks - like the one the world’s in the grip of at the moment - occur, we put in place screening measures to try to prevent people from spreading the problem. At airports, this can include temperature monitoring and questioning about risky symptoms. But how effective is this? As she explains to Chris Smith, Katie Gostic, at the University of Chicago, the answer is quite alarming...
Katie - We were trying to assess how effective traveler screening for this coronavirus might be. That is sort of what fraction of infected people would we expect to be caught if they were screened for fever or exposure risk as they pass through an airport. And we came at this knowing that this sort of traveler screening historically has not been particularly effective for other pathogens that have emerged in the last 20 to 50 years. But we were trying to get a better estimate for how it might play out for this coronavirus specifically.
Chris - What sorts of screening are - and were - people doing?
Katie - So 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," would be one of the biggest risk factors for this pathogen.
Chris - And when you sought to look into this, actually what was your approach?
Katie - The vast majority of screening does consist of this sort of fever and symptoms screening, plus a questionnaire. We came at this using basic probabilistic assumptions. The first thing we did was we asked sort of 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 screening unit 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. So the biggest question that you have to ask first is, is this hypothetical individual that's passing through screening showing symptoms yet or are they still early in the course of infection and haven't yet progressed through the incubation period? And then the second question is, does this person realize that they might have been exposed to the virus? And we basically estimate that people are unlikely to realize 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 - And what data do you then work with? So those are your assumptions, that's how you're approaching it. So do you then take, what, real world travel data, and ask, applied to this, with these assumptions, how many would we find?
Katie - Mostly what we do is we make assumptions and we sort of predict using those assumptions what fraction of cases we would expect to be detected. But 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 we assume that that if infected people passed through that screening and then later developed a severe case, they would eventually appear in the data, basically. Like once someone passes through screening, if they reached their destination and later end up in the hospital, then we know that screening has failed to detect that person.
Chris - Right. And on the basis of your model, in your assumptions, how effective is this screening?
Katie - So we estimate that even in sort of the best case scenario, that screening is probably missing 50% or more of infected travellers with coronavirus. And this is not necessarily an issue with the same machinery used to do screening like thermal scanners or human compliance and implementation, it's more just a biological reality of the way this infection progresses. So we estimate that 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. And we know also for this coronavirus that risk factors are pretty nonspecific. Like essentially anyone who's passed through an area that's known to have an epidemic would be considered at risk. And so the risk questionnaire isn't extraordinarily helpful for detecting cases either.
Chris - Gosh, 50% that's enormous.
Katie - Yes.
Chris - So what are the implications of that?
Katie - Well, we think that this is one of the big reasons that the virus was able to spread so easily at first and one of the reasons that there was such a big delay in realizing that there was community transmission going on in places like Italy or Iran or Seattle before the first cases were detected.
Chris - We are where we are now. We've got a quarter of the world's population as it stands at the moment in late March, currently experiencing some kind of lockdown or restriction on their movements to try to break the transmission chain. 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. And to be honest with you, even when we detect evidence that a new virus is emerging in humans, if it has an animal origin, it's actually surprisingly uncommon for that virus to transmit well from human to humans. And so I think shutting down global air travel networks every time could lead to a lot of false alarms and could really substantially damage the economy. 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 they know from the outset that traveller screening is going to be leaky. And so there's a lot of investment in more on the ground contact tracing - which basically means that public health professionals, as soon as the case is detected, go out and 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, so just because you have a couple of sparks on the ground is not a guarantee that one of those sparks is going to start a larger blaze. And contact tracing is a really good way to deal with the fact that we often can't prevent every spark from coming in, but we can do our best to prevent a big fire from starting.