What Temperature Is Considered a Fever in Adults?

Fevers aren't necessarily a foreign concept: Everyone's experienced their temperature rise at one point or another—mainly due to infections from bacteria or viruses (hello, influenza). But since the onset of COVID-19, there's been quite a bit of confusion as to what those numbers on the thermometer mean—and when they warrant concern.

There are several symptoms of COVID-19, including headache, sore throat, and nasal congestion. Another common symptom is fever. According to an April 2021 systematic review and meta-analysis of 17,515 patients with COVID-19, 79.43% of adults and 45.86% of children presented with a fever with COVID-19. Of the adults who had a fever, 14.71% of them had what is considered a high-grade fever (103 degrees Fahrenheit or above).

It's important to clarify that not all COVID-19 cases will get a fever, nor will everyone with a fever test positive for COVID-19, but it is an extremely common symptom.

What Exactly Counts as a Fever?

According to the US National Library of Medicine's MedlinePlus resource, a fever (aka, pyrexia) is technically a higher-than-normal body temperature. That normal body temperature can vary from person to person but is usually about 98.6 degrees Fahrenheit (though one study claims it has dropped to 97.9°F over the last two centuries).

The Centers for Disease Control and Prevention (CDC) considers a reading of 100.4 degrees Fahrenheit a fever. While an actual temperature reading is the best diagnostic tool, the CDC also says it considers a fever present when a person feels hot to the touch, has previously reported feeling feverish (possibly with chills), or looks flushed or glassy-eyed.

Jill Grimes, MD, FAAFP, a board-certified family physician at UT Austin's Student Health Services, added that there are also different ranges and severities of fevers. A low-grade fever, for example, is used to describe a body temperature that is elevated above normal, but is not above or is just barely above the fever threshold. "So [it's] roughly 99 degrees to 100.9 degrees," said Dr. Grimes, and added that that's "not a true fever."

"In general, when physicians hear that your measured temp is above 101, it catches our attention, meaning we are looking for an infection," explained Dr. Grimes. The American Academy of Family Physicians (AAFP) adds that temperatures of 103 degrees or above are considered high fevers and can signal a potentially dangerous infection that needs medical treatment ASAP.

Why Do Fevers Happen—and What Do They Feel Like?

It's important to note that a fever isn't actually a disease on its own, but a sign that your body is trying to fight an illness or infection, per MedlinePlus. The resource explains that viruses and bacteria that cause infections can thrive in a normal human body temperature environment. A fever results from your body trying to kill the pathogen, through essentially making your body an inhospitable environment for it. Fevers also activate your body's immune system, in an added attempt to kill the pathogen.

As far as what a fever feels like, it's entirely dependent on how high the temperature is, Matthew Mintz, MD, FACP, a board-certified internist practicing in Bethesda, Maryland, told Health. "Feeling warm, flushed, cold, chills, along with general body ache are all common symptoms, though not every patient will get every one of these symptoms," said Dr. Mintz. Dr. Mintz added that fevers can be cyclical, meaning they go up and down. When a fever breaks or comes down, you may also begin to sweat.

Here's where fevers become a little more complicated—at least in regards to COVID-19: With other viruses, like influenza, there's a short period time from when the virus enters your body to the time you start showing symptoms like a fever—and that usually signals to the average person to stay home and get better.

With COVID-19, it takes a little longer for the body to recognize the infection. "When someone contracts coronavirus, symptoms, including fever, can develop within the first two weeks, but on average between four to six days," said Dr. Mintz. "This means that people can actively have coronavirus for several days, potentially infecting other people without knowing they are sick." Dr. Mintz added that, in some cases, fever may not even be an initial symptom—or a symptom at all, adding that some patients will get a cough or other symptoms days before getting a fever, or may be totally asymptomatic.

How Do You Treat a Fever?

In most cases, a slightly elevated temperature isn't necessarily cause for concern. "In general, fevers by themselves are not cause for immediate medical attention in adults unless they persist more than a day or two or are higher than 103," said Dr. Grimes.

According to MedlinePlus, anyone with a fever should rest and stay well-hydrated with water, gelatin, ice pops, or soup. While medication isn't needed for a lower-grade fever, if any fever is accompanied by a severe headache, stiff neck, or shortness of breath, it warrants a visit to the doctor. In fevers above 102 degrees Fahrenheit, acetaminophen (Tylenol), ibuprofen (Advil), or aspirin can be used to reduce the fever, but if it doesn't respond to the medication, or is higher than 103 degrees Fahrenheit, medical attention may be necessary.

In terms of COVID-19 specifically, your temperature alone will not tell you if you are infected; but if you have other common symptoms (dry cough, fatigue, digestive issues) or have severe symptoms (trouble breathing, pressure in chest, confusion, inability to stay awake, blue or gray tint to skin, lips or nail beds) it's important to contact your doctor and get tested. In the event that you are concerned about a potential COVID-19 diagnosis, you should begin self-isolating immediately. And if you are eventually diagnosed with COVID-19, it's important to follow your doctor's instructions, which include further self-isolation, self-management of symptoms, and seeking help if your symptoms or condition worsens.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDC, WHO, and their local public health department as resources.