Tuesday, April 29, 2014

MERS: The Limitations Of Airport Screening



Scheduled airline traffic around the world, circa June 2009 – Credit Wikipedia



# 8548


As the graphic above illustrates, airline traffic is a very effective conduit for infectious disease, able to transport someone to nearly anyplace in the world in less than 24 hours.  The world’s airlines carry 2.6 billion passengers each year, on more than 17 million flights.


In 2003, we saw the SARS virus hop the Pacific on a flight taken by a 78-year-old woman who fell ill in Toronto after visiting Hong Kong, and before that outbreak was contained, 251 people in Canada had been infected, and 44 died (see SARS And Remembrance).


So, whenever a virus threatens to spread globally one of the first visible actions taken by many governments is to impose some sort of airport, seaport, or border screening to prevent those who are infected from entering the country. 


Over the weekend we saw a declaration of an emergency in all of Egypt’s ports after a MERS case was detected coming in from Saudi Arabia - and quarantined - at the Cairo Airport last Friday.  Today, we are seeing media reports of two more `suspected cases’ being tested, but it is unknown if they are actually MERS cases.


Yesterday Malaysia announced an intensive airport screening program for passengers arriving in Kuala Lumpur  (see Visitors entering Malaysia to be screened for Coronavirus). 


While certainly reassuring to the public, and likely to pick up at least some symptomatic carriers of an emerging virus, studies to date indicate the effectiveness of airport screening to be pretty limited.


One of the big problems lies in the incubation period, that for many viral infections, allows a long `asymptomatic’ window during which someone can travel before showing symptoms. The incubation period of seasonal influenza runs 1 to 4 days (avg. 2 days), measles 7 to 21 days (avg. 14 days), MERS-CoV up to 15 days. 


Plenty of time for someone to pass through airport screening, and travel in-country for several days, before showing any outward symptoms of infection.

While we don’t know if asymptomatic carriers of the MERS virus are infectious, we do know that with influenza it is possible to pass on the virus while not showing signs of illness.  So even those who are infected, but never show signs of illness, may be capable of spreading the virus. 


In Japan: Quarantine At Ports Ineffective Against Pandemic Flu  I wrote about a study that suggests between asymptomatic or mild infections, and a silent incubation period of several days, there wasn’t much chance of long-term success.

For every person identified, and quarantined, by port authorities  - researchers estimate 14 others infected by the virus entered undetected.

And in 2009, during the initial outbreak of H1N1, we saw airline passengers taking fever-reducers to beat the airport scanners in order to get home (see Vietnam Discovers Passengers Beating Thermal Scanners).


All `holes’ in the screening process that would allow infected travelers to enter a country undetected.


Between the SARS outbreak of 2003 and the 2009 pandemic, we’ve a number of studies that have looked at just how effective airport screening is in a `real world situation’. 

Last year, Helen Branswell reported on the value of airport screening in an article called:


Airport disease screening rarely worthwhile, study suggests

Helen Branswell, The Canadian Press
Published Wednesday, April 10, 2013 10:11AM EDT

  • TORONTO -- A new study suggest airport screening for disease control rarely makes sense, but if it's undertaken, it should be done at the source of the outbreak.

    The researchers say the screening of passengers leaving via a few key airports near the epicentre of an outbreak is a better approach than having hundreds of airports around the world screen arriving passengers.

    (Continue . . . )


    Politically, and in terms of reassuring the public, the screening of passenger arriving at airports and other points of entry probably has some merit.  And it may provide valuable surveillance information as well.  But practically, as an effective way to keep an emerging virus out of a country, studies continue to show just how unlikely that outcome really is.


    We simply possess no technological shield that would keep an emerging pandemic virus at bay. 


    Making it desirable that – whenever possible – outbreaks of emerging viruses are quashed as quickly as possible at the source, before they can board an airplane and spread inexorably around the globe.