Study: Effectiveness Of Non-Contact Infrared Temperature Screening
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HEAT SEEKER (Time Magazine 2003)
The idea is appealing.
Using non-contact infrared scanning technology, you could theoretically scan the queues of people entering airports, or exiting airplanes, or in almost any venue (office building, hospital, etc.) and detect those who are running a fever and who might be carrying a communicable disease.
NCITs (non-contact infrared thermometers) were deployed during the SARS outbreak in 2003, and a number of airports around the world have either installed them, or are considering doing so, in anticipation of a future influenza pandemic.
But the question is, do they really work?
In an attempt to answer that question a new study has appeared in this week's edition of Eurosurveillance:
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
ABSTRACT (reformatted for readability)
Several countries plan to introduce non-contact infrared thermometers (NCIT) at international airports in order to detect febrile passengers, thus to delay the introduction of a novel influenza strain.
We reviewed the existing studies on fever screening by NCIT to estimate their efficacy under the hypothesis of pandemic influenza. Three Severe Acute Respiratory Syndrome (SARS) or dengue fever interventions in airports were excluded because of insufficient information. Six fever screening studies in other gathering areas, mainly hospitals, were included (N= 176 to 72,327 persons; fever prevalence= 1.2% to 16.9%).
Sensitivity varied from 4.0% to 89.6%, specificity from 75.4% to 99.6%, positive predictive value (PPV) from 0.9% to 76.0% and negative predictive value (NPV) from 86.1% to 99.7%. When we fixed fever prevalence at 1% in all studies to allow comparisons, the derived PPV varied from 3.5% to 65.4% and NPV was >=99%.
The low PPV suggests limited efficacy of NCIT to detect symptomatic passengers at the early stages of a pandemic influenza, when fever prevalence among passengers would be =<1%.
External factors can also impair the screening strategy: passengers can hide their symptoms or cross borders before symptoms occur. These limits should be considered when setting up border control measures to delay the pandemic progression.
Helen Branswell of the Canadian Press, one of our favorite science reporters, has a review of these findings where she explains the limitations of the technology.
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.
The authors, from France’s Health Watch Institute, said the available scientific data suggests there is little benefit to airport temperature screening when the incidence of disease is low, as it was with SARS and as it would be expected to be in the very early days of a pandemic.
“Because public perceptions are important, policy-makers may feel some pressure to use NCIT” — non-contact infrared thermometers — “but the decision making process should not ignore the poor scientific evidence on NCIT’s efficacy to delay the introduction of a novel influenza strain,” they wrote.
Should we see another SARS outbreak, or the start of an influenza pandemic, nearly every nation will see public pressure to shut down airports, seal their borders, and `keep the virus out' of their country.
Unfortunately it isn't as easy as it sounds.
Since SARS and Influenza both have an incubation period where the carrier can be infected, but not showing symptoms, temperature screening at airports is likely to miss those who are in the early stages of infection.
And there are limits to the technology as well. The study makes the following points that lend to a lack of sensitivity of the NCITs.
- 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.
The authors sum up their report with this:
Because public perceptions are important, policy makers may feel some pressure to use NCIT but the decision making process should not ignore the poor scientific evidence on NCIT's efficacy to delay the introduction of a novel influenza strain.
For transparency reasons, the surrounding sociological, demographic, epidemiological and environmental factors which can influence the screening strategy must also be taken into consideration.
So while the idea has a great deal of appeal, this study calls into question the usefulness of temperature screening, particularly if the goal is to keep pandemic flu carriers out of a country.
