Can You Tell If It's the Flu or COVID-19? Doctors Say It's Not So Clear

Updated November 2024

With influenza and the usual cold-weather respiratory infections in circulation now, along with COVID-19, it's not always easy to figure out what's ailing you if you get sick.

We talked to UC San Francisco emergency care physician Jahan Fahimi, M.D., and infectious disease specialist Peter Chin Hong, M.D., about the differences between flu and COVID-19, when to get a test, and why you shouldn't try to diagnose yourself.

Note: If you think you're experiencing a life-threatening or severe condition, call 911 or go directly to the nearest emergency department.

Is it possible to tell the difference between flu symptoms and COVID-19 symptoms?

"It can be challenging, because the flu, COVID-19 and other respiratory illnesses can have a variety of overlapping symptoms," said Fahimi. Those may include fever, chills and body aches, upper respiratory symptoms like runny nose and sore throat, lower respiratory symptoms like cough and pneumonia, and gastrointestinal symptoms like nausea, vomiting and diarrhea.

"While certain symptoms are slightly more associated with one virus than the other, there's enough overlap that there's uncertainty," cautioned Fahimi. "So, we wouldn't use the presence or absence of those symptoms to rule either illness in or out."

Typical flu symptoms are relatively consistent: fever, cough and muscle aches. Those are also common in COVID-19, but COVID-19 symptoms vary more wildly than those of the flu, from no symptoms at all to deadly pneumonia and myriad cardiovascular and neurological issues, said Chin-Hong.

"Influenza is engineered to cause disease, so once you get it, you're going to have symptoms and be stuck in bed," said Chin-Hong. "COVID depends on what soil it lands on. Once you get COVID, you can have a wide range of consequences."

Does COVID-19 have any telltale symptoms?

One symptom that does seem more specific to COVID-19 is the sudden loss of smell or taste. It was more common in the early years of the pandemic and is reported less frequently now – but you should definitely call your provider if you or someone in your family experiences it.

"That's one symptom that would make me more likely to suspect COVID-19," said Fahimi.

Chin-Hong noted that some symptoms in isolation – such as allergy-like symptoms, like a runny nose and watery eyes, or diarrhea alone – may now be the only COVID-19 symptoms you have. Children under 10 are less likely to be infected with and show symptoms of COVID-19, so if they have flu-like symptoms, they're more likely to have the flu, he added.

But these are generalizations. Thanks to the variable nature of COVID-19, the only way to know whether you're infected is to be tested.

"I can't give you a magic formula," said Chin-Hong. "At the end of the day, it's really testing that's going to tell you."

What should I do if I feel sick?

If you feel unwell and have any of the symptoms mentioned above, follow standard rules for preventing the spread of illness, such as washing your hands, wearing a mask, staying home from work, and avoiding close contact with people in your household. In addition, you should take an at-home COVID-19 and/or flu test.

If you test positive for COVID-19, follow the precautions mentioned above to avoid spreading it. If you are at high-risk, you can also take an antiviral like Paxlovid and follow up with your doctor if your condition gets worse.

Flu tests – which used to require a visit to your doctor or urgent care – are now sold in drugstores and online as part of a combination flu and Covid test. When diagnosed early, the flu can be treated with the antiviral Tamiflu.

Even if you test negative for both COVID-19 and flu, try to limit your exposure to others or wear a mask until your symptoms disappear, said Fahimi. There's a small chance that a negative test result will turn out to be wrong. Moreover, many other respiratory illnesses – from the common cold to croup – circulate during flu season, and spreading them spreads uncertainty.

"Everyone's focused on the flu and COVID-19, but there's a whole host of other stuff going around that can present very similarly," said Chin-Hong.

When should I seek medical attention right away?

If you experience shortness of breath, seek medical care right away, said Chin-Hong. You may notice, for example, that it's harder to catch your breath walking up stairs. That could be a sign that your oxygen levels are low, possibly due to COVID-19 or flu.

If you're older or immunocompromised, or you have other health conditions that make you more vulnerable to COVID-19, contact your doctor "sooner than later" if you have any symptoms, said Chin-Hong.

Do COVID-19 and the flu spread differently?

Both flu and COVID-19 are spread mainly by respiratory droplets launched via coughs and sneezes. But COVID-19 can also be transmitted more stealthily, said Chin-Hong, through smaller droplets emitted when we talk or sing ("aerosols") that can linger in the air.

Also, people with COVID-19 can transmit the virus up to two days before they show symptoms and even if they never develop symptoms. Many people with COVID-19 infections are "asymptomatic," meaning they have no symptoms. In contrast, said Chin-Hong, most people with the flu are well aware that they have it.

"That's why COVID-19 is so insidious. People don't even know they are infected, and they can spread it," he said.

What action can I take now?

The best way to avoid the "What do I have?" question this winter is to get the flu and COVID-19 shots, wear masks if needed and wash your hands.

"We want to reduce the number of people getting these symptoms and having to make these decisions in the first place, because they're going to be difficult," said Chin-Hong. "My advice is: Don't even put yourself in that position."