Currently, in a matter of hours, air travel can transport passengers—and the bugs they carry—from one continent to another. The H1N1 virus and Ebola are just two of the many diseases whose spread was fueled by international airline travel. Therefore, screening passengers at airports, could be one important method for slowing the global spread of infections. A research team led by UCLA scientists has found that airport screening misses at least 50% of infected travelers; however, the researchers note that the rate could be improved. They published their findings were published on February 19 in the journal eLife.
For the study, the investigators used a mathematical model to analyze screening for six viruses: the Ebola virus, the SARS coronavirus, the Marburg virus, the Middle East respiratory syndrome coronavirus, Influenza H1N1, and Influenza H7N9. “We found that for diseases with a long incubation period, such as Marburg and Ebola, taking passengers’ temperature to test for fever is particularly ineffective at the start of an epidemic but does pick up more cases as the epidemic stabilizes,” explained lead author Katelyn Gostic, a UCLA doctoral student in the laboratory of senior author James Lloyd-Smith, PhD, a UCLA associate professor of ecology and evolutionary biology. She added, “With diseases such as swine flu, which take a shorter time to incubate, fever screening is the most effective method throughout an epidemic.”
Typically, airport personnel conduct screenings before passengers board their flights, when they land at their destinations, or both. The study authors note that fever screening on arrival has been criticized for being ineffective; however, it can detect cases that are missed before passengers’ flights depart. The screeners often use infrared non-contact thermometers to identify passengers with a fever; however, previous studies have shown that the devices identify fevers only about 70% of the time, thus, the “double-check” of arriving passengers can help detect individuals who were missed before their departures.
At present, traveler questionnaires are one of the tools screeners use; for example, they query passenger whether they have been in contact with an infected individual (in the case of Ebola) or have handled live poultry (for viruses such as avian influenza). The investigators note that screeners could more effectively identify ill passengers if those who create those questionnaires understand the risk factors for each disease; this would lead to better designed surveys. An obvious problem with the questionnaire is that infected passengers are likely to lie, fearing that they will be banned from the flight or quarantined upon arrival. The investigators found that only about 25% of passengers answered honestly regarding whether they had been exposed to influenza during the 2009 pandemic; furthermore, some passengers may have hidden their symptoms by taking medication.
“Anyone who reports honestly puts himself or herself at risk of delay or detainment; this is a terrible incentive for truthful reporting,” noted Gostic. She added, “A high number of people use over-the-counter drugs like acetaminophen that conceal fevers and can make their symptoms undetectable, which is likely an overlooked problem.” Lloyd-Smith added, current screening programs can reduce the rate of importing infections, but nowhere close to zero. Even under the best-case scenarios we considered, arrival screening missed at least half of infected travelers for all pathogens. Traveler screening by these methods is inherently leaky.”
The authors had some suggestions to improve screening. Lloyd-Smith explained, “An important gap is that we have little direct data on the efficacy of departure screening. This is needed to weigh the benefits of different screening policies and areas for investment. For example, in the current Ebola outbreak, how many potential travelers were turned away before boarding airplanes to depart West Africa? Of these, how many were actually Ebola cases? There is broad agreement that departure screening is probably more efficient than arrival screening, but we don’t actually have any examples where we know how well it worked in practice.”
The investigators recommend that cost-effectiveness studies should be conducted that would enable policy makers the ability to assess the social and economic impact of screening policies at departure and arrival; however, they noted that these studies will require more extensive data on the efficiency of current screening practices. They also recommend studies to quantify how many travelers are using antipyretics (fever-lowering drugs), and evaluate the possible use of incentives to encourage honest reporting.