# 3024
Appearing in Travel Medicine and Infectious Disease this month is a study on our ability to screen incoming air travelers with Pandemic Influenza.
First the abstract (hat tip Florida1 on FluTrackers), then some discussion.
U.S. airport entry screening in response to pandemic influenza: Modeling and analysis
Travel Medicine and Infectious Disease, In Press, Corrected Proof, Available online 14 April 2009,
John D. Malone, Robert Brigantic, George A. Muller, Ashok Gadgil, Woody Delp, Benjamin H. McMahon, Russell Lee, Jim Kulesz and F. Matthew MihelicSummary
Background
A stochastic discrete event simulation model was developed to assess the effectiveness of passenger screening for Pandemic Influenza (PI) at U.S. airport foreign entry.
Methods
International passengers arriving at 18 U.S. airports from Asia, Europe, South America, and Canada were assigned to one of three states: not infected, infected with PI, infected with other respiratory illness. Passengers passed through layered screening then exited the model. 80% screening effectiveness was assumed for symptomatic passengers; 6% asymptomatic passengers.
Results
In the first 100 days of a global pandemic, U.S. airport screening would evaluate over 17 M passengers with 800 K secondary screenings. 11,570 PI infected passengers (majority asymptomatic) would enter the U.S. undetected from all 18 airports. Foreign airport departure screening significantly decreased the false negative (infected/undetected) passengers. U.S. attack rates: no screening (26.9%–30.9%); screening (26.4%–30.6%); however airport screening results in 800 K–1.8 M less U.S. PI cases; 16 K–35 K less deaths (2% fatality rate). Antiviral medications for travel contact prophylaxis (10 contacts/PI passenger) were high – 8.8 M. False positives from all 18 airports: 100–200/day.
Conclusions
Foreign shore exit screening greatly reduces numbers of PI infected passengers. U.S. airport screening identifies 50% infected individuals; efficacy is limited by the asymptomatic PI infected.
Screening will not significantly delay arrival of PI via international air transport, but will reduce the rate of new US cases and subsequent deaths.
While there will likely be public clamor to try to block the entry of a pandemic virus into this, or any other country, the truth is – for most of the world it probably isn’t remotely possible.
A person can be infected with PI (Pandemic Influenza) and not show any symptoms for several (1-4) days. Plenty of time to fly in from anyplace in the world, and defeat any screening process.
Areas that receive a small number of arrivals might be able to institute a quarantine system (see Can Island Nations Effectively Quarantine Against Pandemic Flu? ), but even then the ability to interdict infected travelers won’t be 100%.
The four successful quarantines during the 1918 pandemic were in American Samoa (5 days' quarantine) and Continental Australia, Tasmania, and New Caledonia (all 7 days' quarantine).
- The Spanish Flu did not reach American Samoa until 1920, and had apparently weakened, as no deaths were reported.
- Australia's quarantine kept the influenza away until January of 1919, a full 3 months after the flu has swept New Zealand with disastrous effects.
- Tasmania kept the flu at bay until August of 1919, and health officials believed they received an milder version, as their mortality rate was one of the lowest in the world.
- By strictly enforcing a 7-day quarantine, New Caledonia managed to avoid introduction of the virus until 1921.
Eventually, once the quarantines were lifted, the virus did make it to these isolated regions of the world.
Quarantines for air and sea ports that handle a substantial number of passengers really isn’t practical. It is here that screening passengers for fever and flu-like symptoms might be attempted.
There are, however, serious limitations in detecting potentially infected travelers.
HEAT SEEKER (Time Magazine 2003)
Earlier this year a study by French researchers (see Study: Effectiveness Of Non-Contact Infrared Temperature Screening) appeared that indicated that the infrared temperature screening devices being deployed into airports were of limited value.
Helen Branswell, of the Canadian Press, wrote the following:
Studies show little merit in airport temperature screening for disease
Monday, 16 February 2009 - 11:58am.
By Helen Branswell
TORONTO — Using temperature scanners in airports to try to identify and block entry of sick travellers during a disease outbreak is unlikely to achieve the desired goal, a report by French public health officials suggests.
Their analysis, based on a review of studies on temperature screening efforts like those instituted during the 2003 SARS outbreak, says the programs may be of limited use in the early days of a flu pandemic, when governments might be tempted to order screening of incoming travellers to try to delay introduction of the illness within their borders.
Still, there are compelling reasons to at least try to limit the number of infected people arriving during a pandemic – particularly in the opening days and weeks of an outbreak.
Each infected person that arrives can start a new chain of infection. Each person they infect can go on to infect others. The more starting foci of infection, the faster the virus will spread.
Since it takes time to produce a vaccine (months), and time to prepare a society to deal with a pandemic, any delaying action that can reduce the speed and spread of the virus has value.
No, it won’t stop the virus from arriving. But it could help delay the spread of the virus.
And in a severe pandemic, a delay of a few weeks could save thousands of lives.
