A fresh study from the BMC Infectious Diseases journal on the efficacy of thermal scanners used to detect H1N1 influenza among arriving passengers at Japan’s Narita International Airport during the 2009 pandemic.
As you are probably aware, many airports installed thermal scanners during the SARS outbreak of 2003 and during the pandemic of 2009, in an attempt to identify and interdict infected passengers upon arrival.
(Thermal Imaging in 2003)
Since fever is often a hallmark of infection, thermal imaging has been promoted as a way to protect the public and (hopefully) delay introduction of a virus into a country during a pandemic.
While all of this sounds reasonable, their usefulness has been the matter of considerable debate for years.
The problem is, not everyone who is infected will exhibit a fever.
- Some may be silently incubating the virus, and will become symptomatic in another 24-48 hours
- Others may have other symptoms, but no fever
- Some may be taking antipyretics (fever reducers) to ease symptoms or evade detection
- And some may simply be asymptomatic carriers of the virus.
Added to these, scanners can be foiled by other factors including the consumption of hot beverages or alcohol, pregnancy, menstrual period or hormonal treatments. All of which can increase the external skin temperature and cause a false positive.
Inversely, intense perspiration or heavy face make-up can have a cooling effect on the skin temperature which can cause a false negative.
Bitar , A Goubar, J C Desenclos
In June of 2009, just as the pandemic was ramping up, I wrote Vietnam Discovers Passengers Beating Thermal Scanners, which looked at a Reuters report that a number of sick passengers flying into Ho Chi Minh City in Vietnam took fever reducers (Aspirin, Tylenol, etc) several hours prior to arrival in order to beat the thermal scanners.
Epidemiology of travel-associated pandemic (H1N1) 2009 infection in 116 patients, Singapore. Emerg Infect Dis 2010 Jan; [e-pub ahead of print]. Mukherjee P et al
Airport thermal scanners detected only 12% of travel-associated flu cases; many travelers boarded flights despite symptoms.
Travelers play a key role in spreading many infections, including influenza. Such was the case with the spread of 2009 H1N1 influenza to Singapore, a major travel hub serving 37 million air passengers annually.
One quarter of patients were symptomatic when they boarded flights; 15% developed symptoms during travel. Airport thermal scanners detected only 12% of patients overall and only 40% of those with symptomatic infection on arrival.
Thermal scanners for screening travelers do moderately well at detecting fever, but do a poor job at flagging influenza, according to researchers from New Zealand who presented their findings today at the International Conference on Emerging Infectious Diseases (ICEID) in Atlanta.
They assessed the performance of the machines from Aug 21 through Sep 12, 2008, on 1,275 passengers arriving from Australia at Christchurch International Airport. The investigators took the travelers' tympanic temperatures and obtained respiratory samples for influenza testing from 1,268 of them.
The positive predictive value for fever was 1.5% for thermal scanners and 4.1% for tympanic thermometers. For influenza, the positive predictive value for the two techniques was 2.8%. None of the 30 passengers who tested positive for influenza had a tympanic temperature of 37.8°C (100°F) or higher, and only 2 had temperatures of at least 37.5°C (99.5°F). Three were asymptomatic. The group concluded that fever is a poor predictor of influenza, which limits the efficacy of thermal screening at entry points.
Jul 13 ICEID abstracts (See Board 168)
These older less-than-encouraging findings all serve as prelude to today’s study, which finds similar limitations with the use of thermal scanners to detect influenza during a pandemic.
Detection rates of H1N1 were found to be just over 22% and thermal imaging sensitivity for detecting fevers ranged from just over 50% to just under 70%.
I’ve only reproduced the abstract. Follow the link to read this open access study in its entirety.
Hiroshi Nishiura and Kazuko Kamiya
BMC Infectious Diseases 2011, 11:111doi:10.1186/1471-2334-11-111
Published:3 May 2011
Entry screening tends to start with a search for febrile international passengers, and infrared thermoscanners have been employed for fever screening in Japan. We aimed to retrospectively assess the feasibility of detecting influenza cases based on fever screening as a sole measure.
Two datasets were collected at Narita International Airport during the 2009 pandemic. The first contained confirmed influenza cases (n=16) whose diagnosis took place at the airport during the early stages of the pandemic, and the second contained a selected and suspected fraction of passengers (self-reported or detected by an infrared thermoscanner; n=1,049) screened from September 2009 to January 2010. The sensitivity of fever (38.0 C) for detecting H1N1-2009 was estimated, and the diagnostic performances of the infrared thermoscanners in detecting hyperthermia at cut-off levels of 37.5 C, 38.0 C and 38.5 C were also estimated.
The sensitivity of fever for detecting H1N1-2009 cases upon arrival was estimated to be 22.2% (95% confidence interval: 0, 55.6) among nine confirmed H1N1-2009 cases, and 55.6% of the H1N1-2009 cases were under antipyretic medications upon arrival. The sensitivity and specificity of the infrared thermoscanners in detecting hyperthermia ranged from 50.8-70.4% and 63.6-81.7%, respectively. The positive predictive value appeared to be as low as 37.3-68.0%.
The sensitivity of entry screening is a product of the sensitivity of fever for detecting influenza cases and the sensitivity of the infrared thermoscanners in detecting fever. Given the additional presence of confounding factors and unrestricted medications among passengers, reliance on fever alone is unlikely to be feasible as an entry screening measure.
Despite the aggressive use of thermal imagers, passenger interviews, and other screening methods - Japan found it impossible to prevent entry of the H1N1 virus into their country during the early days of the 2009 pandemic.
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.
First a article from The Yomiuri Shimbun, followed by a link to the study which appears in Eurosurveillance.
The Yomiuri Shimbun
The number of people infected with new flu who passed undetected through airport quarantine between April last year, when the new strain of influenza began spreading, and May was about 14 times higher than the number of infected people stopped at the airports, according to a study carried out by Tokyo University.
Eurosurveillance, Volume 15, Issue 1, 07 January 2010
H Sato , H Nakada, R Yamaguchi, S Imoto, S Miyano, M Kami
Politically, and in terms of reassuring the public, the deployment of thermal scanners at airports and other points of entry probably has a lot of merit. And they can provide valuable surveillance information as well.
Practically, as an effective way to keep an emerging virus out of a country, these studies continue to show how unlikely that outcome really is.