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Tuesday, April 23, 2013

In pandemics, screen passengers at exit points

No matter where in the world the next epidemic starts, it’s no more than a day’s plane ride from reaching anyone on the planet. Needless to say, doctors are keen to stop diseases from spreading around the globe. To that end, Kamran Khan of St. Michael’s Hospital, Toronto, and his colleagues have a proposal: screen airline passengers as they depart from risky areas.

Screening airline passengers for infectious diseases may be a tempting idea in principle. In practice, it’s more complicated. Leaving aside the issues of privacy and expense, it could add to already tedious boarding procedures and lead to huge disruptions in travel. However, if we do choose to implement screening practices, there are better and worse ways to do it.

The most inefficient health screening checks are done at the point of entry. Most passengers arriving at an airport were never exposed to any dangerous diseases. Even people arriving from an at-risk location might not have been there long enough to contract anything. For example, take air travelers leaving Mexico during the 2009 H1N1 influenza pandemic. The researchers estimated that catching any possible influenza carriers arriving from Mexico would have required screening stations at 82 international airports spread through 26 countries. 116 people would have been screened at those entry points for every person with any possibility of being infected. And that’s just counting direct flights. Adding in all passengers traveling through Mexico on connecting flights would have meant screening 67 million people in over a thousand airports. Clearly, entry screening is not the way to go.

How about screening people as they leave a hazardous region? Again, referring to the H1N1 pandemic, the researchers concluded that exit screening at just eight Mexican airports would have caught 90% of the people who could have been carrying the disease. Clearly, if you can choose where to conduct your health screenings, exit points are the way to go.

There are a couple of problems with this strategy. The most glaring one is that the scientists don’t say how air passengers should be screened. How do you process that many people in a way that doesn’t unduly slow down air traffic without missing anything? Also, screening only at exit points means that other countries have to trust each other’s screening procedures. Plus, there’s always the risk that a person will not begin to show symptoms until after departing from the exit airport. Finally, these check points will most likely only be used if there is some indication that an epidemic may be brewing. By that time, at least a few contagious passengers will undoubtedly have already been transported to new regions.

Khamran Khan, Rose Eckhardt, John Brownstein, Raza Naqvi, Wei Hu, David Kossowsky, David Scales, Julien Arino, Michael MacDonald, Jun Wang, Jennifer Sears, & Martin Cetron (2013). Entry and exit screening of airline travellers during the A(H1N1) 2009 pandemic: a retrospective evaluation Bulletin of the World Health Organization BLT.12.114777.