Are temperature scanners just a lot of hot air?
During SARS, H1N1 influenza, Ebola virus disease (EVD) and now the COVID-19 pandemic, there has been widespread uptake of temperature scanners to detect persons affected by these viral infections and prevent them from entering workplaces, public facilities, boarding airplanes or crossing a border. But is the use of temperature scanners to detect a fever an effective means of preventing transmission of infectious diseases?
How temperature scanners work
The human body emits infrared (IR) radiation at wavelengths that can be detected by a temperature scanner and converted into an estimate of body temperature. Temperature scanners can be handheld devices for individual temperature measurement (non-contact infrared thermometers (NCITs)) or thermal scanning cameras (TSCs) for screening in large public places.
An elevated body temperature is an indicator of fever, which could be an indicator of infection. Normal human body temperature is about 37°C and the threshold temperature for fever detection using temperature scanners is typically around 37.5-38°C. Detection of a temperature above a fever threshold can trigger immediate exclusion or result in secondary screening that may include health-related screening questions and a confirmatory temperature reading. The outcome of secondary screening may be exclusion from entry or travel, a recommendation that the individual to seek medical attention or a recommendation to quarantine.
How have they been used in previous pandemics?
During SARS, H1N1 influenza, Ebola virus disease (EVD) and dengue fever pandemics, temperature scanners were used for entry and exit screening at border crossings and international airports. Numerous evidence reviews assessing the effectiveness of temperature scanning have found that it has been an ineffective measure in past epidemics for identifying infected individuals at airports or border entry and exit points. (see links 1, 2, 3, 4, 5, 6, 7) In the SARS pandemic of 2003, only 191 of over 700,000 travellers screened at Toronto and Vancouver airports were referred to secondary screening and no confirmed cases of SARS were detected. Similarly, of over 160,000 travellers screened at Freetown airport in Sierra Leone between September 2014 and February 2016, just 10 cases were referred to secondary screening but none were found to be positive for EVD. During this time, it was later revealed that two cases of EVD had travelled through the screening process undetected. There have been examples of individuals using antipyretics or other medication to reduce a fever to evade detection by temperature scanners to avoid potential travel delays or exclusion from work in some countries.
Not all studies have found a lack of ability to detect infected persons. A study of temperature screening for incoming cases of dengue fever at Taiwan airports between 1998 – 2007 found that airport screening identified 45% of imported cases. This study also compared the overall effect on community transmission with and without airport temperature scanning. No significant benefit was detected, potentially due to the small number of total cases detected at airports (244) and the many imported cases that went undetected.
Temperature Scanning in the COVID-19 pandemic
During the COVID-19 pandemic the use of temperature scanners has gone far beyond border entry and exit points. Across the globe, These devices are being used for daily screening of students and staff entering schools, for persons entering homeless shelters, for screening residents and visitors to long-term care facilities, and workers in food processing plants and various other workplaces. The US Centres for Disease Control has outlined considerations for temperature checks as part of health screening for staff and children in child care programs, schools and day camps, workplace screening for workers at high risk, and employees of restaurants, bars, mass transit and long term care facilities. In Canada, the use of temperature screening varies by jurisdiction and local epidemiological conditions. In many settings, temperature screening is implemented as a precautionary measure, but without much consideration of its effectiveness in preventing transmission.
Why use temperature scanners?
Temperature scanners can provide a quick, non-contact means of quickly assessing persons before they enter a facility. The technology is low-cost, easily accessible and requires minimal training. Stopping people for a temperature check can also provide an opportunity to ask screening questions about symptoms or provide reminders on COVID guidelines such as hand sanitizing and physical distancing. While limited value has been identified for temperature screening at airports or borders, there has been much less assessment of its effectiveness in other settings such as for schools or workplaces. One example from the Cargill meat processing facility in Colorado indicated that temperature screening was effective in preventing a symptomatic person, who was later confirmed to have COVID-19, from entering the workplace and infecting others. This person later died from COVID-19. The exclusion of this worker potentially limited transmission to others
The technology has been found to significantly out-perform self-reporting in identifying fever in a clinical setting, which suggests that questionnaires, or self-assessment forms alone could fail to identify fever. There is also some suggestion that temperature scanners may provide a deterrent for symptomatic persons considering entering a workplace or venue, but there is limited evidence to quantify this supposition.
What are the limitations of temperature scanners?
Temperature scanners can not provide a diagnosis and should not be used to influence treatment decisions. Compared with traditional thermometers, some temperature scanners have variable performance and show poor correlation with traditional thermometers. A review of effectiveness found a high incidence of both false positive (up to 25%) and false negative (up to 20%) detection of fever for some scanners.
The World Health Organization has stated that temperature screening at entry or exit points is not an effective method to limit international transmission of SARS-CoV-2. There are many reasons why temperature scanners can be ineffective:
- Fever may not be present in pre-symptomatic or asymptomatic persons, and people can be transmitting the virus before they show symptoms.
- COVID-19 symptoms are variable, with studies indicating that fever may occur in as few as 44% of cases.
- Persons whose fever may have subsided can continue to shed virus.
- Fever or elevated temperature can also be due to non-COVID illness, other physiological conditions (e.g., pregnancy, hormonal treatments, hypothyroidism), or environmental conditions (e.g. ambient temperature, humidity, time of day).
- Variations in how the scanner is used (e.g., distance to the sensor, location of test) may affect the accuracy of a reading.
The epidemiological situation in the community may influence the likelihood of detecting fever. In the outbreak at the Cargill plant in Colorado, widespread local transmission increased the likelihood that workers entering the plant were potentially infected. Where community spread is low, and uncertainty in accuracy is high, there may be less benefit from scanning.
Temperature scanning could present a false sense of security that a facility is free from COVID due to these entry checks. Temperature scanning should not be relied on over other measures such as questionnaires on health and travel status, emphasis on self-reporting of illness, adhering to physical distancing recommendations, wearing of masks where appropriate, and good hand hygiene. The practice should also not increase risk of transmission to the person taking the temperature or between persons being scanned. Those performing scans should be trained to use the equipment safely and appropriately. Locations recording temperature results along with personal information about health status or symptoms, such as workplaces, should be aware of any data and privacy obligations when collecting medical information about employees.
- Temperature scanning has been ineffective in previous pandemics in detecting infected persons and there is limited evidence that temperature scanning prevents community spread of infectious diseases.
- Temperature scanners may provide an opportunity for health screening and informing individuals about various public health measures to reduce transmission.
- Venues, facilities, and workplaces using temperature scanners should be aware of their limitations and ensure that other proven control measures are implemented first.