Traveller screenings may miss 50% of cases
Screening programs have been deployed at travel hubs across the globe to limit the spread of the 2019 novel coronavirus (2019-nCoV), but just how effective are they? To estimate the impact of different travel screening programs, researchers at Bat One Health examined the values of key nCoV life history and epidemiological parameters, which included known incubation period, rate of infection, sensitivity of thermal scanners for fever detection, and time from symptom onset to patient isolation.
Their findings were that, even under the best-case assumptions, “with just one infection in twenty being subclinical and all travellers passing through departure and arrival screening”, nearly two-thirds of infected individuals will not be detected. Researchers attributed the findings to two main factors. The first fundamental challenge was that screening depended on individuals showing detectable symptoms, which increased with time since exposure. This meant that many infected individuals would travel during their incubation period, at a point where they felt healthy enough to travel but were simultaneously most difficult to detect. Second, their models showed that effective screening depended on whether questionnaires were sufficiently sensitive to detect exposure risk factors. For nCoV, because the evidence for specific risk factors has been limited, “it was assumed that at most 40% of travellers would be aware of a potential exposure, and that a minority would self-report their exposure honestly, which led to limited effectiveness in questionnaire-based screening.” The convergence of these two factors resulted to many infected travellers being fundamentally undetectable.
The authors wrote that these findings have implications for assessing when different programs were worthwhile investments. They recommended that in areas that have yet to experience community-based transmission of the virus, arrival screening where travellers are “provided with an information card to self-screen and self-report, alongside increased general surveillance and awareness in healthcare settings,” could delay importation of cases and build awareness among incoming travellers.” They advise against taking “false confidence from reports that infected travellers are being detected by existing screening programs,” and underscore that “for every case detected by travel screening, one or more infected travellers were not caught, and must be found and isolated by other means.”
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