An early sign of many infections is a fever or an increase in your core temperature that your body uses to help fight pathogens. Researchers have found the no-contact forehead thermometers commonly used to screen for fever in public places are an ineffective and unreliable means of detecting COVID-19.1
Thermoregulation is an important part of maintaining a homeostatic environment in the body. Homeostasis is a physiological state in which an organism seeks stability or equilibrium between interdependent systems. Your body temperature is integral to creating equilibrium for other systems to optimally function. The entire system is complex and sometimes may seem counterintuitive.2
Your body can use negative and positive feedback loops3 to help regulate your core temperature within a narrow range, irrespective of the external environment.4 Infection is another reason your body's core temperature can rise. The primary virus of interest in 2020 has been SARS-CoV-2 that triggers COVID-19 infection.
The first confirmed case in the U.S. occurred January 21, 2020, when the Centers for Disease Control and Prevention found a Washington state resident who had recently returned from Wuhan, China, was infected.5 By January 23, 2020, Wuhan was under quarantine and shortly thereafter the U.S. declared a public health emergency.
The early unfolding events of the COVID-19 pandemic included recommended measures to help reduce the spread of an unknown virus. These included social distancing, sheltering in place, mask-wearing and temperature taking.
Skin Temperature Can Change Independent of Core Temperature
The body maintains an internal temperature that varies less than 1.8 degrees Fahrenheit (1 degree Celsius) throughout the day, depending on your physical activity, foods you've eaten or emotional state.6
A change in core body temperature outside this range can happen during an illness or when the conditions in the external environment are more than your body is able to handle, such as extreme heat or extreme cold. In high heat, the body will shunt blood to the skin to increase sweat production and release heat, which assists in thermoregulation.
This variability in skin temperature independent of your core body temperature is one reason the no-contact forehead scanners are unreliable. While fever is a common symptom of COVID-19, the scanners can result in false negatives and false positives.
Researchers in physiology are suggesting that using the same scanners to assess a person's temperature at the fingertip and eye would offer more reliable readings.7 The study was published in Experimental Physiology and co-led by physiologist and expert in thermoregulation, Michael J. Tipton, Ph.D., who is also editor-in-chief of the same journal.8
While reviewing the science behind temperature taking, the writers suggested infrared thermography forehead scanners were of limited use for mass screening.9 As outlined in a press release from the Physiological Society, the paper determined four key factors that relate to using the scanners:10
- Not all people who have the virus have a fever, so temperature alone is not a good indicator.
- Skin temperature can vary from body core temperature and therefore is not an accurate estimate, and a direct measure of core temperature is impractical.
- Not every person with a fever, even an accurate core body temperature, has COVID-19.
- Using temperature assessments from the finger and the eye may offer more reliable information of an increase in body core temperature.
Tipton Proposes Adapting Infrared Thermography Scanners
In their review of early data, the researchers found the most common symptom in 55,924 patients with confirmed cases who presented in China through February 22, 2020, was fever. In this group, at least 11% did not have a fever and fewer than half with suspected disease who were admitted to the hospital had a fever.11
The press release cites a 2005 study in which forehead temperatures were compared across three infrared forehead thermometers in 1,000 participants. The measurements varied by as much as 3.6 degrees F (2 degrees C).
In another study with 500 participants, more than 80% of the infrared measurements produced a false negative result. There is a range of reasons that thermometers can produce false-negative results, including:12
Adipose levels in the body
Distance from the scanner
Tipton commented on the results of the study and how the information could be used, saying:
“If scanners are not giving an accurate reading, we run the risk of falsely excluding people from places they may want, or need, to go, and we also risk allowing people with the virus to spread the undetected infection they have. Using a surface temperature scanner to obtain a single surface temperature, usually the forehead, is an unreliable method to detect the fever associated with COVID-19.
Too many factors make the measurement of a skin temperature a poor surrogate for deep body temperature; skin temperature can change independently of deep body temperature for lots of reasons. Even if such a single measure did reflect deep body temperature reliably, other things, such as exercise can raise deep body temperature.
We think we can improve the identification of the presence of fever using the same kit but looking at the difference between eye and finger temperature – it’s not perfect, but it is potentially better and more reliable.”
Not All People With COVID-19 Have a Fever
As Tipton pointed out from data gathered early in the pandemic through February 2020, not all people who have COVID-19 will have a fever. As more data have been gathered throughout 2020, the number of people who may have an infection, but present without fever, has grown. Your body produces a fever with infection since most viruses and bacteria find it harder to survive in higher temperatures.13
As with other types of infection, people may experience a range of mild to severe disease. The CDC lists several symptoms associated with COVID-19 that may appear within two to 14 days after being exposed to SARS-CoV-2. Among these are fever, cough, shortness of breath, fatigue and a new loss of taste or smell.14
The difference in the symptoms a person may experience is also related to whether they have a mild, moderate or severe infection. To date, there has not been a definitive study on the prevalence of individuals with COVID-19 who will also present with a fever.
Research published in the Journal of the American Medical Association offers a clue that a fever is not as common with mild illness as was originally believed.15 The researchers evaluated the symptoms of 202 participants who completed the study. They found that an alteration in taste or smell was one of the first symptoms reported in patients who had mild or asymptomatic disease.
They cautioned that the sample was geographically limited and patients with severe disease were not included. A second later study from South Korea showed in 213 people with confirmed cases of COVID-19, the most common symptom was a cough followed by a loss of taste and smell.16 In this group, fever was only recorded in 11.6% of the individuals.
Normal Temperatures Have Dropped Below 98.6
German doctor Carl Reinhold August Wunderlich collected data from 2,500 people, amounting to millions of temperatures.17 Whether people were sick or healthy, he recorded their body temperatures and then analyzed and published the data in the mid-1800s. This data helped establish a normal temperature of 98.6 Fahrenheit (37 Celsius), which medicine uses as one factor in a health evaluation.
However, research published in 2020 found that human core body temperatures in the U.S. have been declining through the Industrial Revolution.18 Dr. Julie Parsonnet from Stanford University department of medicine and her team gathered more than 677,000 temperature measurements from nearly 190,000 people collected from 1862 to 2017.
The data were split into three chronological groups and analyzed, leading the team to conclude that body temperatures have dropped an average of 1.06 F in men and 0.58 F in women over a 200-year time period.19 To minimize potential measurement bias, the scientists also compared temperature readings within a specific population. Commenting on the results of the study, Parsonnet said:20
“Our temperature’s not what people think it is. What everybody grew up learning, which is that our normal temperature is 98.6, is wrong. Physiologically, we’re just different from what we were in the past.
The environment that we’re living in has changed, including the temperature in our homes, our contact with microorganisms and the food that we have access to. All these things mean that although we think of human beings as if we’re monomorphic and have been the same for all of human evolution, we’re not the same. We’re actually changing physiologically.”
While the results are interesting, Kenneth Welch, Ph.D., who was not involved in the study, believes it will not make a difference in the day-to-day life of individuals.21 Even infections that trigger a small rise in body temperature normally also trigger body aches, chills or headaches commonly associated with a fever. Rising temperatures from other environmental factors or even allergies do not usually trigger the same experience.
How to Take an Accurate Temperature
When you have a cold, flu or other infectious condition, such as COVID-19, it is always best to steer clear of others and reduce the spread of infection. After all, no one really enjoys being sick.
Other than body aches, chills and generally feeling miserable, you have several options for determining if you have a fever at home.22 A digital thermometer can be used in the mouth, rectum or under the arm. Purchase disposable protective sleeves to help keep the thermometer clean and make sure you're not passing germs from one person to another.
If you plan to use both oral and rectal temperatures, use separate thermometers for each and be sure they're labeled accordingly. When taking an oral temperature, wait at least 15 minutes after you've eaten or drunk anything to avoid an inaccurate reading.
A tympanic (ear) thermometer takes a digital reading from inside the ear canal and must be positioned correctly to get an accurate reading. Ear wax and small ear canals can interfere with accuracy. To use an infrared scanner, the thermometer must be placed over the temporal artery to get an accurate reading.
The type of thermometer you choose may not be as important as following the directions to achieve an accurate measurement and ensure you don't pass germs with each use. It is crucial you do not rely strictly on temperature to determine the severity of an illness. Additional symptoms, such as dehydration, lethargy and confusion, are strong indicators of illness and must be considered.