An illustration of a fever thermometer.

When it comes to COVID-19, the disease caused by the novel coronavirus, it turns out not much is simple. Not even fever. 

Yes, it's clear that a fever, along with fatigue and a dry cough, is a hallmark of the infection. But how high, exactly, is too high? What should you do if your temperature spikes? How do you know when you need medical help?

As someone who has reported on healthcare for years, I thought the answers to these questions would be relatively straightforward. But after speaking to experts, I came to learn that in the context of COVID-19, how to respond to fever varies based on the person and the circumstance.

Here’s what you need to know.

Why We Develop Fevers

We tend to think of normal body temperature as a static number, but in fact a person’s body temperature can vary throughout the day, says Georgine Nanos, M.D., a family physician and CEO of the Kind Health Group, a telemedicine service. Nor is there a single number that constitutes “normal” for everyone. Recent research has even found that the average body temperature has changed over the years. Scientists say it is now around 97.5° F, not 98.6° F.

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A fever is technically defined as a body temperature of 100.4° F or higher, according to the Centers for Disease Control and Prevention. This is true for both babies and adults.

And a fever isn’t necessarily a bad thing. “It indicates that your body is doing what it should do in response to an infection,” Nanos says. Indeed, it’s a sign that your immune system is doing its job, fighting off an invader, like a virus or bacteria, in an attempt to prevent it from overtaking your body. 

But a fever can be concerning. Aside from how miserable it may make you feel—with sweating, chills, headaches, and fatigue—it could indicate that a serious illness is developing. And an uncontrolled fever could cause seizures or brain damage. So it's important to pay attention to fevers and respond appropriately. 

What the Thermometer Number Means

There are different types of thermometers and ways to use them. Most used today are digital and can be used orally, rectally, or under the arm. But there are also some designed to be used in ears, called tympanic thermometers, or on your forehead, called temporal artery thermometers. 

More important than which kind you use is using it according to the directions that come with it, experts say. 

They also caution that you shouldn’t get too focused on the exact number on your thermometer. "A temperature in the low 99s could be a cause for concern in some people, while someone with a fever of 102 may be feeling and doing okay," says Mary E. Schmidt, M.D., an associate professor of clinical medicine at Virginia Commonwealth University in Richmond and president of Schmidt & Libby Health Advisory Group.

What’s more, how healthcare professionals treat fever depends on age, accompanying symptoms, and any individual’s underlying conditions.

For example, the American Academy of Pediatrics recommends that you call your doctor right away if your infant 3 months and younger develops a fever of 100.4° F. (Rectal readings are the most accurate for babies and tend to be a degree higher than oral readings.)

For babies older than that and for adults, there is no exact reading that dictates that you need to call the doctor. Instead, you should consider your overall health along with your temperature reading.

So, for instance, an otherwise healthy adult with a fever of 101° F and only mild discomfort may be just fine at home with rest and fluids. But an adult with a fever of 101° F who also has heart disease and diabetes—particularly if the fever is accompanied by severe aches and chills—should call a doctor for guidance.

Fevers and COVID-19

At this point in the COVID-19 outbreak, scientists have not identified a specific fever pattern tied to this disease.

The same is true of the illness’s other main symptoms. “People are reporting an array of symptoms with varying degrees of severity, so there is no one set of signs that can tell us for sure, ‘yes or no, this is COVID-19,’” says Elissa Perkins, M.D., associate professor of emergency medicine at Boston Medical Center who is treating the influx of patients there. “It would make everybody's lives much easier if that were possible, but it’s not.”

To decide who may need to be seen by a healthcare professional or tested for COVID-19, Perkins says doctors evaluate a fever along with other symptoms, a person’s age, health history, and any underlying conditions. Then, based on the capacity of local hospitals and the limited availability of tests and other supplies, public health professionals may advise people with symptoms of COVID-19 to stay in touch with a doctor on the phone rather than go directly to the emergency department.

“We are giving our best advice based on what we know, and how somebody is doing right now,” Perkins says.

What to Do If You Don't Have a Thermometer

One of the issues confronting people during this outbreak is that many physical and online stores are sold out of thermometers.

If you don’t have one, the CDC says that “a fever may be considered to be present if a person . . . feels warm to the touch or gives a history of feeling feverish.” In other words, Mom’s method of placing the back of her hand on your forehead may be a decent fallback. But given the contagiousness of this coronavirus, remember to wash your hands thoroughly afterward.

And don’t worry if you don’t have someone at home who can do a touch check of your forehead. Schmidt notes most adults are fairly good at gauging when they’re feeling hot and unwell.

You could also look in the mirror. The CDC states that “the appearance of a flushed face, glassy eyes, or chills” could indicate a fever.

As a last resort, if you have a meat thermometer (preferably a digital one, which shows decimal points), you could experiment by placing it under your tongue to get a general sense of your temperature, and whether it’s going up or down. Though the method is certainly unorthodox, and not specifically recommended, I asked two doctors to test it out and they kindly obliged.

Schmidt says that her “meat thermometer read about the same as an oral thermometer over three takes, but was 1 to 1.5 degrees lower than a temporal artery reading” via her forehead. Nanos also humored my request and says that her meat thermometer “is too variable, and the temperature drops as soon as you start pulling it out of your mouth.”

I gave my meat thermometer a try, too. I found that it gave a comparable reading to my oral thermometer. It took a couple of minutes for the meat thermometer to show a significant rise, but it indicated that I was in the 98 range, just like my oral thermometer reading.

Granted, the meat thermometer reading was not exact and it did drop down as soon as I took it out of my mouth, but if you’re trying to track trends in your temperature and you have no other option, that meat thermometer could be better than nothing.

If you do call a doctor to report your temperature, be sure to note which type of thermometer you used. Healthcare professionals know that different types of thermometers tend to yield slightly different numbers, so they will want to consider that information in assessing how you’re doing.

How to Treat a Fever If You Suspect COVID-19

There is no rule that says you need to lower a fever with over-the-counter medication if you’re feeling only mild symptoms and are not uncomfortable. You may be able to manage with rest (for fatigue), a cold compress (for sweating), blankets (for chills), and lots of liquids (for dehydration). 

There is even evidence behind the adage that you should let a fever run its course. Animal studies have found that a fever may help the immune system do its job, and lowering a fever with medication may suppress your body’s ability to fend off the illness, says Matthew J. Kluger, Ph.D., a scientist and retired professor of physiology at the University of Michigan in Ann Arbor.

Kluger's groundbreaking research in the 1970s suggests that fever is an adaptive biological response to infection and it may boost the effectiveness of disease-fighting cells. "Treating a fever with medication may make you feel better, but my contention is that it's not because it lowers your temperature; it's because the medication is an analgesic, a pain reducer," Kluger says.

For the most part, healthcare providers say that if you’d like to take a fever-reducer (and it does not interact with any medications you’re taking or worsen any underlying conditions you may have), by all means do so, because it could make you feel more comfortable.

But if your fever is running at or above 103° F, you should call a doctor. A high fever could lead to a seizure or brain damage. 

Tylenol vs. Advil

There are two main categories of over-the-counter drugs available to reduce fevers: acetaminophen, most well-known by the brand name Tylenol; and a class of drugs called nonsteroidal anti-inflammatories, or NSAIDs, which includes ibuprofen (Advil and others), naproxen (Aleve and others), and aspirin. 

In the past weeks, there were reports out of China and Europe suggesting that NSAIDs could make a COVID-19 infection worse. As a result, some healthcare providers had suggested avoiding NSAIDs for fevers during this time. 

But experts say that theory is based on anecdotal data, and that for now, there is no reason to avoid ibuprofen or other NSAIDs. The Food and Drug Administration, for example, says “it is not aware of scientific evidence connecting the use of NSAIDs, like ibuprofen, with worsening COVID-19 symptoms.” And the World Health Organization notes that “based on currently available information, WHO does not recommend against the use of ibuprofen. We are also consulting with physicians treating COVID-19 patients and are not aware of reports of any negative effects of ibuprofen, beyond the usual known side effects that limit its use in certain populations.”

Perkins, at Boston Medical Center, says that concerns about ibuprofen and NSAIDs could reflect the lack of solid medical research available on the quickly developing health problem. "There’s just a paucity of good, reliable data to inform the medical decisions that we make now, so that is a challenge we’re all facing when we try to decide how to manage these patients,” Perkins says. Still, she has not yet seen any compelling reason to avoid NSAIDs like ibuprofen if you suspect COVID-19.

When to Get Medical Help

If you think you have a temperature that is worrisome, the first step in most cases is to call a healthcare provider. That's because a high temperature on its own might not warrant an in-person evaluation right now, depending on your overall condition and risk factors.

“We’re telling patients that if you have mild symptoms and no underlying medical conditions, stay home and self-isolate,” for 14 days, Nanos says. “If you can, have one person care for you so that you will minimize exposure to others in the house." After 14 days, the CDC says you can discontinue home isolation if you've been fever-free for 72 hours and it's been at least seven days since your first symptoms appeared.

Go to the hospital or urgent care clinic if you develop any of these emergency warning signs for COVID-19, per the CDC: trouble breathing, persistent pain or pressure in the chest, bluish lips or face, or new confusion or fainting.

Perkins reiterates the importance of seeking medical help if you experience emergency warning signs, and says that if you can, “call the emergency department to tell them you are on your way, so they can prepare for you.”