Half of coronavirus cases are missed in 'inherently leaky' screenings: study

As world leaders and health officials try to cope with the rapid spread of the novel coronavirus, COVID-19, new research warns people that less than half of cases are actually detectable through traveller screenings.

“We estimate that even in sort of the most optimistic scenarios that our model considers, screening is going to miss at least half of infected travellers,” Katelyn Gostic, a postdoctoral fellow at the University of Chicago and one of the authors of this new study told Yahoo Canada.

“Our analysis shows that screening is missing these travellers not because of human error, but because they are just fundamentally undetectable, because they don’t show symptoms yet.”

The study, published earlier this week in the journal eLife, uses a mathematical model developed in 2015 that estimates the effectiveness of travel screenings to prevent the spread of pathogens. It found that the amount of time it takes for an individual to show symptoms makes screening for the virus, both before and after departure, difficult.

“Screening is inherently leaky and public health policy makers know this, this is not new to coronavirus,” Gostic said. “I think that screening is something that gives people sort of a sense of security early in an outbreak, it’s a very public measure that feels like a little bit of a security blanket.”

The challenges with traveller screening

Travellers fall into one of four categories for screenings: symptomatic but unaware of exposure; aware but asymptomatic; symptomatic and aware; and neither symptomatic nor aware.

Screenings include risk screening questionnaires, which ask travellers if they have been in an area with known transmission or have come in close contact with someone who has been confirmed, or is a suspected, coronavirus case. Thermal scanners are also used as a way to check for fevers.

In terms of why so many cases can go undetected, Gostic said the primary challenge is a large number of travellers not showing detectable symptoms.

There are two reasons why symptoms may not show at the time of the screenings. One is related to the incubation period, someone who may have been exposed hasn’t developed any symptoms yet, while travellers could also just have a mild case of the virus.

“It’s sort of a Catch-22, always in epidemiology, that the more mild cases are really difficult to detect because those people are just much less likely to show up at the doctor, where they can get tested and confirmed positive for a given disease,” Gostic said.

Another challenge is related to the questionnaire, which doesn’t ask the specific questions required to identify particularly high risk individuals who may not show symptoms.

“With coronavirus it seems like there might be substantial fractions of people who don’t even realize they’ve been exposed,” Gostic said. “You can’t fall back on just asking people questions about their behaviour if they don’t yet show symptoms.”

The difficulty present in detecting cases of COVID-19 is evident in cases like the Diamond Princess cruise ship, where people are quarantined, being monitored, and the virus spread quickly.

“We know that large fractions of the cases that are being diagnosed in those settings don’t show symptoms at the time of diagnosis, and so, that’s sort of a worst case scenario in some ways because the cases in quarantined are being actively monitored,” Gostic said. “They’re likely to be diagnosed relatively early in the course of infection but it definitely illustrates the challenge of detecting cases.”

Although there are inherent problems with traveller screenings, Gostic said the end goal is to prevent the start of a local chain of transmission, even if some cases arrive at the destination. This includes efforts on the group to follow up with close contacts of confirmed cases and getting people into isolation quickly. But she also stressed that changes in simple behaviours can significantly decrease the transmission potential.

“Just simple things like washing your hands or staying home from work or school or public events if you feel sick can make a really big difference,” Gostic said.

“Just because screening isn’t effective, it doesn’t mean that people should be panicking, there are other measures in place to try to stop or slow the spread of this disease and there are sort of basic behavioural changes that people can make in their lives that can really have an impact.”