During the global pandemic caused by the coronavirus, temperature screening emerged as a widespread non-pharmaceutical intervention (NPI) adopted globally by governments, businesses, and institutions. This measure involved checking individuals for an elevated body temperature before allowing them entry into public spaces, such as airports, schools, and workplaces. The primary goal was to rapidly identify and isolate people infected with the SARS-CoV-2 virus, thereby limiting disease spread. While temperature checks were implemented on a massive scale, their utility in effectively containing the pandemic became a subject of considerable debate. Limitations soon became apparent as the nature of the virus and flaws in the screening technology were better understood.
The Rationale Behind Temperature Screening
The initial adoption of temperature screening was rooted in the physiological response of the human body to a viral infection. A fever, generally defined in a non-healthcare setting as a temperature of 100.4°F (38.0°C) or higher, is one of the earliest and most frequent signs of systemic illness. This elevated temperature is a biological defense mechanism where the body resets its internal thermostat to create an environment less favorable for the pathogen.
Public health officials focused on fever because it is an objectively measurable symptom that could be quickly checked in high-traffic areas. The epidemiological theory was that by capturing symptomatic individuals at points of entry, they could be diverted for further testing and isolation, preventing them from transmitting the virus to others. This straightforward approach was appealing due to its low cost, speed, and perceived effectiveness as a first line of defense. Similar temperature checks had been used in previous outbreaks, including the Severe Acute Respiratory Syndrome (SARS) epidemic in 2003. The expectation was that a significant portion of infected individuals would present with fever, making it a reliable proxy for active infection.
Methods and Devices Used for Measurement
The need for rapid and non-invasive screening methods led to the mass deployment of specialized instruments designed to take a person’s temperature without physical contact. The most common tool used in public settings was the Non-Contact Infrared Thermometer (NCIT), often referred to as a forehead scanner or “temperature gun.” These handheld devices were used by screeners to take a reading at the forehead or temple from a short distance.
Larger public venues, such as airports and corporate lobbies, often utilized thermal imaging cameras, or thermal scanners. These systems were capable of scanning multiple people simultaneously as they walked past, displaying a live thermal image that could flag individuals with elevated surface temperatures. Both NCITs and thermal cameras operate on the same fundamental principle of detecting infrared radiation. Every object with a temperature above absolute zero emits thermal radiation, and the intensity of this radiation increases with temperature. The NCIT contains a lens that focuses the infrared energy onto a detector called a thermopile. This component converts the thermal energy into an electrical signal, which the device then translates into a numerical temperature reading displayed on the screen. The device measures the temperature of the skin surface, which is used as an estimate of the body’s core temperature.
Technical Limitations Affecting Accuracy
The effectiveness of temperature screening was consistently hampered by technical limitations related to the devices and the method of measurement itself. Non-contact infrared thermometers (NCITs) are designed to measure the temperature of the skin surface, which is not the same as the body’s internal, or core, temperature. This reliance on surface readings introduces inherent variability and potential inaccuracies into the screening process.
Environmental factors can significantly skew the readings from NCITs, leading to false results. For instance, a person coming from a cold outdoor environment or standing under an air conditioning draft may register a falsely low reading, even if they have an internal fever. Conversely, direct sunlight, high ambient heat, or recent physical exertion can elevate the skin’s surface temperature, resulting in a false positive reading.
Physiological factors also impact measurement accuracy; surface blood flow, the presence of sweat, or wearing a hat can all interfere with the infrared emission from the skin. Even when used correctly, the difference between an NCIT reading and a more accurate measure of core temperature can have a wide limit of agreement, sometimes ranging over two degrees Fahrenheit. Furthermore, the lack of proper calibration or inconsistent use, such as holding the device at the wrong angle or distance, compounds these measurement errors.
The diagnostic performance of temperature screening, measured by its sensitivity and specificity, was poor for detecting COVID-19 infection. Sensitivity refers to the tool’s ability to correctly identify a person who has the infection. Studies showed that the sensitivity of temperature-based screening for SARS-CoV-2 infection was extremely low, with some analyses placing the range between 0% and 40.5%. While specificity—the ability to correctly identify a healthy person—was generally high, the low sensitivity meant that many infected individuals were incorrectly cleared through screening.
Evaluating Overall Public Health Effectiveness
Temperature screening ultimately proved to be an ineffective standalone strategy for controlling the spread of the coronavirus. The primary reason for this failure was biological and epidemiological: the nature of SARS-CoV-2 transmission bypassed the screening method’s capabilities. A significant percentage of infected individuals, estimated to be up to 45%, were asymptomatic, meaning they carried and transmitted the virus without ever developing a fever or other noticeable symptoms.
Moreover, the virus was transmissible during the pre-symptomatic phase, before any symptoms like fever had a chance to manifest. An infected person could therefore pass the screening checkpoint, only to become symptomatic and highly infectious a day or two later. This high rate of transmission by people without a fever rendered the screening measure useless for a substantial portion of the contagious population.
Even among those who did develop symptoms, fever was not a universal or constant sign of infection; it is estimated that between 13% and 50% of symptomatic cases may never present with a fever. Compounding this issue, individuals could easily mask an emerging fever by taking over-the-counter antipyretic medications, such as acetaminophen, before going through a checkpoint. Ultimately, data from real-world work settings showed the limitation of the strategy, with one study reporting that for every case of COVID-19 identified by temperature screening, approximately 40 cases were missed. This evidence indicated that the measure had negligible value as a disease control method and served more as a visible reassurance or psychological deterrent, rather than a reliable public health tool.