(Thermal Imaging in 2003)
The idea sounds simple.
By screening passengers for fever when they arrive via airplane (or boat or train) from another country you can hopefully reduce the number of infected passengers that enter during a pandemic.
In reality, it isn’t simple at all.
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.
Before the 2009 H1N1 virus entered Singapore, public health officials implemented a containment plan, in which all passengers arriving at the international airport underwent thermal scanning, and all passengers with influenza-like illness (ILI) were referred to a designated screening center (Tan Tock Seng Hospital [TTSH]) for treatment and isolation.
In this report, investigators described the first 116 patients hospitalized at TTSH with travel-associated H1N1 infection. By definition, all patients arrived in Singapore during the containment phase of the epidemic, developed ILI within 10 days of arrival, and had H1N1 influenza confirmed via real-time reverse-transcriptase PCR on respiratory samples.
The first patient arrived in Singapore on May 26, 2009, and the others followed over a 5-week period. During the first 2 weeks, most patients had acquired their infections in the U.S., but the origin of exposure shifted rapidly to Australasia and Southeast Asia. The mean age of patients was 29, and half were Singaporean.
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. At the time of evaluation, only 61% of patients had fevers high enough to meet CDC criteria for ILI, and 54% had fevers that met the WHO criteria.
If all of this sounds vaguely familiar, it may be because the subject of the efficacy of thermal scanners has come up before.
In June of this year, just 60 days after the first novel H1N1 cases began to be noticed in Mexico and California, we learned that travelers to Vietnam were finding ways to beat the thermal scanners installed at their airports.
The desire to get home when you are sick, or even when you fear you might be starting to fall ill, can be enormous.
Home is familiar.
Home is where your family, your friends, and your doctor are close at hand. Home is where people speak the same language, and have the same customs.
If you are going to be sick, the last place you want it to be is in another city . . .or worse, another country.
For some, there is the extra complication that their medical insurance may not be valid outside of their own country as well.
And so it was probably naïve not to expect that some people during a pandemic wouldn’t try every trick in the book to get home. Even if they suspected they might be infected.
Today, we learn from this 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.
Aside from the problem of deliberately taking fever reducers in order to beat the scanners, since SARS and Influenza both have an incubation period of (generally) 1 to 4 days, it is very likely that visitors could arrive before they show signs of a fever.
Additionally, some individuals may carry the infection asymptomatically.
And there are limits to the technology as well, as we learned from a report that appeared in Eurosurveillance last February.
Bitar D, Goubar A, Desenclos JC. International travels and fever screening during epidemics: a literature review on the effectiveness and potential use of non-contact infrared thermometers. Euro Surveill. 2009;14(6):pii=19115. Available online
The study referenced above makes the following points that lend to a lack of sensitivity of the NCITs (non-contact infrared thermometers).
- Individual factors such as the consumption of hot beverages or alcohol, pregnancy, menstrual period or hormonal treatments can increase the external skin temperature.
- Inversely, intense perspiration or heavy face make-up can have a cooling effect on the cutaneous temperature without a parallel decrease of the actual body temperature.
- The targeted body area scanned by the detector also plays a role, because of physiological differences in vascularisation and consequently in heat distribution.
Helen Branswell of the Canadian Press summarized the report in her article:
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.
This latest study doesn’t completely dismiss the use of thermal scanning, but it does caution that only limited success can be expected with its use.
Our data show that >30% of case-patients from all flights >3 hours had symptom onset before arrival, but overall, only 12% of all case-patients were detected by thermal scanners, suggesting that thermal scanners detected 40% of those symptomatic patients.
This early detection and isolation may still have a valuable adjunctive role, especially in the initial phase of outbreaks.
Situations favoring the use of airport thermal scanners include short-incubation diseases and geographically distant outbreak epicenters, such that arriving passengers have been on a long-haul flight.
However, if the converse were true, with transmission occurring in nearby countries and passengers arriving from short-haul flights, symptoms would develop in most passengers who become ill after entry and, thus, would be missed by airport thermal scanners.