Yes or no? On or off? Zero or one? Binary is simple, and simple is good. It facilitates decision-making, especially in a crisis like a pandemic. After all, either you have COVID-19 or you don’t. If you have COVID, then you are infectious and should isolate to avoid spreading it. On the other hand, if you don’t have COVID, you can’t infect anyone else, no matter how closely you associate with them. Of course, the tricky part is determining whether or not someone has COVID.

The PCR test is the gold standard for determining if a person has COVID-19. It’s a very good test that gives us the yes-or-no binary information that we value so much for making decisions. Unfortunately, the test is not always readily available and it’s also expensive. And timing is critical. If you take the test too soon after you are infected, the virus may not have yet traveled to your nose where the sample is taken, and thus the result may be a false negative — you have COVID but the test indicates you don’t. Also, it often takes time in a laboratory to process the results — will you isolate or carry on while you’re waiting?

COVID Failure: A Matter of Principle


Finally, what would prompt you to get a COVID test? Perhaps some event prompts you or requires a test by policy, but otherwise, you might take a test because you feel sick. If so, you already know you may be infectious. In that case, a positive COVID test merely confirms what you already suspect, and you normally get that confirmation a couple of days too late to do any good. Despite our heavy reliance on testing, it’s not as simple or as timely as we would like for deciding when to isolate.

We’ve had another way to separate the healthy from the sick during the COVID-19 pandemic: symptoms. For example, if you have a fever, then you may be infectious. But temperature-based screening has not been very effective at all, and a big reason why is that the US government has historically defined fever as 100.4°F (38°C) or above. If a person’s body temperature is 100.3°F, then according to the government, that person does not have a fever. Does that make sense?

Unfortunately, one of the distinguishing characteristics of COVID is the tendency of many infected people to have mild or even unnoticeable symptoms, including only slightly elevated body temperature, below 100.4°F. So, the government’s definition of “fever,” although simple and binary, has only confused the situation. Some people who were asymptomatic with COVID-19 took their temperature, found it to be below 100.4°F and assumed they did not have a fever. So, they carried on with normal day-to-day activities, often infecting others. Temperature-based screening systems typically use the government’s 100.4°F fever threshold, and, as a result, failed to prevent entry by many infected persons. Relying on the government’s 100.4°F fever definition has contributed to the spread of COVID-19. Where did this government standard come from, how can it be improved, and why has the US resisted change?

© Douglas Dyer

Origins of 100.4°F

In 1868, a German physician, psychiatrist and medical professor named Carl Reinhold August Wunderlich published a paper describing his assessment that normal body temperature is relatively constant, varies from 97.9°F to 99.3°F (36.6°C to 37.4°C), and averages 98.6°F (37°C). He found that patients with a disease often exhibited a symptom of fever that he found to average at or above 100.4°F. He based these findings on 1 million temperature measurements for 25,000 patients.

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For the time, this scientific result was quite remarkable, and it changed medicine forever because it gave physicians the newfound ability to objectively assess the presence and severity of many diseases. However, Wunderlich’s patients were mostly German rather than being from different cultures, his thermometer may have been less accurate than those we have today, and people are a little different now than they were then.

These are reasons to suspect that Wunderlich’s ideas of normal body temperature and fever are somewhat different today than they were in the mid-1800s. But, to be fair, Wunderlich observed differences in temperature based on many variables when healthy, and he advised that temperature averages have many “shades of gray.” In particular, Wunderlich noted that even smaller rises in temperature are cause for concern, and that there is no definite temperature threshold over which a person transitions from health to sickness. He said that any “elevation of the axillary [under the arm] temperature above 99.5°F (37.5°C) or any depression below 97.2°F (36.5°C) is always very suspicious.” He added: “But even when every precaution has been taken in making the observations, it is impossible to draw a hard and fast line to indicate by temperature the exact limits of health and disease.”

© Douglas Dyer

Today, clinical research suggests that Wunderlich’s findings should be revisited, that the normal temperature range varies by the individual, and that there is no arbitrary fever threshold that works for everyone. Yet, the US government and some medical experts still regard 98.6°F as normal body temperature and 100.4°F or above as a fever. For COVID019, this is simple, easy and, for most people, wrong.

Improving on 100.4°F as a Fever Threshold

If you’re interested in seeing if 100.4°F is an appropriate fever threshold for you, try taking your temperature. Use a normal, digital, under-the-tongue thermometer for at least 60 seconds. Make sure you haven’t consumed anything for 15 minutes — a hot or cold drink or food will change your measurement. Keep your mouth closed during the reading. Assuming you are healthy, if your temperature is below 98.6°F, then it’s a good bet that your fever threshold is under 100.4°F.

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If you were to take your temperature every day, preferably in the morning when you first wake, you would see that your normal temperature varies in a range of one degree or so. For example, in the image below is the normal temperature data for a person we’ll call JRDA5.

© Douglas Dyer

From this graph, we can see that JRDA5’s normal body temperature varies from 96.6°F to 97.4°F when healthy, and you can expect your own normal temperature to vary also.

In modern medicine, a fever is understood to be a temperature elevation above a person’s normal range. This definition of fever is more accurate than an arbitrary fever threshold like 100.4°F that is based on population averages and data from 150 years ago. A person’s normal temperature range depends on many factors such as age, sex, nutrition and level of activity, and so different people will have different fever thresholds.

Almost always, a fever threshold defined as above your normal temperature range is below 100.4°F. Therefore, if we use this new definition, there is significant potential for identifying sick people using temperature-based screening. Relying on 100.4°F is insufficient for identifying mild, pre-symptomatic or asymptomatic cases of COVID-19.

Why the Government Has Resisted Changing the Definition of “Fever”

A pandemic is not the best time for complicated methods. Perhaps the US government chose to stick with 100.4°F for simplicity and consistency. But, in this pandemic, nothing has been simple. We’ve learned to take advantage of vaccines that need boosting, tests that need repeating and symptoms that keep changing. People can figure out their normal temperature range and their own personal fever threshold if that means effective screening. Having a fever or not is still binary, even if we define fever as above your normal range. It’s still pretty simple.

Elevated temperature is not definitive proof you have COVID-19. We all like certainty, and the PCR test will remain the gold standard for COVID. But we don’t need certainty to make a decision to isolate. A fever should prompt isolation, even though it may not be caused by COVID. The next step is to get tested and then wait for the results. We can stop the pandemic if people isolate if they get a fever. Fever is the most timely indicator we may be infectious.

Asymptomatic cases may not exhibit any elevated temperature, so we cannot depend on temperature screening anyway. It’s possible that there are some people infected with COVID-19 who do not have any fever, perhaps because their immune system doesn’t work at all. However, we know that many asymptomatic cases are accompanied by elevated body temperature lower than 100.4°F. We can catch those people using the more correct definition of fever. The perfect should not be the enemy of the good.

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People hate change and the government is no different. It takes a lot to pass federal legislation and to modify federal regulations. But the government’s 100.4°F fever threshold isn’t working. The effort to change will help us control the pandemic.

How Redefining “Fever” Helps

Since the omicron variant of COVID-19 emerged, we’ve seen increased demand for testing, with many people standing in line for hours waiting to get a test. In the United States, the government has been ordering more tests to address the shortages. However, the demand for testing can evidently overrun our testing resources. By using a more accurate definition of “fever,” people will have a better idea of when they need to get tested. Today, about 75% of tests come back negative. We have clinical evidence that fever and other readily available health data can predict test results. By redefining “fever,” we can make testing more efficient.

We can also monitor our health every day, conveniently, in our own homes. We can’t afford to give everyone a daily PCR test, and hardly anyone wants that anyway. In contrast, it’s easy, fast and affordable to take our temperature every day. It’s a smart, safe way to help keep our friends and family safe and do our part to fight the pandemic. A lot of people would self-monitor if they knew it would help.

The coronavirus that causes COVID-19 evidently mutates easily, giving rise to variants, and we don’t expect that to change. It’s possible there are already variants that are not caught by current tests. Redefining “fever” can help identify cases that PCR tests miss. So far, fever is a symptom of all variants. More broadly, fever is a symptom of many other infectious illnesses, such as the flu. Isolating when you have a fever is appropriate for new variants and other viruses to help prevent the spread and keep everyone safer.

It’s high time for the government to redefine “fever” as body temperature above a person’s normal, healthy range. With a more accurate definition, temperature-based screening can be a powerful new tool for fighting the pandemic — and one well-suited to use by anyone, at home and in time to make a difference. Americans want to help fight the pandemic. It’s about time the government helps them do just that.

The views expressed in this article are the author’s own and do not necessarily reflect Fair Observer’s editorial policy.