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

By Nina Bai

Woman blowing nose and holding mug

As the flu season ramps up this fall and winter, every cough and sniffle will carry an extra worry: possible COVID-19 infection.

With influenza, COVID-19, and the usual cold-weather respiratory infections in circulation, experts say it’s bound to be a confusing season for people to sort out what to do when they fall ill.

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

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

“I think it’s tough because both the flu and COVID-19 can have a variety of overlapping symptoms,” said Fahimi. Those symptoms can include fever, chills and body aches, upper respiratory symptoms like runny nose and sore throat, lower respiratory symptoms like cough and pneumonia, and some gastrointestinal symptoms like nausea, vomiting and diarrhea.

“While you could say 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 in or out either illness.”

The typical symptoms of flu are relatively consistent – fever, cough and muscle aches. These are also common in COVID-19, but it’s become clear as the pandemic has progressed that COVID-19 symptoms vary more wildly than those of the flu – from no symptoms at all in some 45 percent of cases to deadly pneumonia and myriad cardiovascular and neurological issues, said Chin-Hong.

“Influenza is engineered to cause disease, so once you get influenza, you’re going to get 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.”

Are there any symptoms that are distinctive about COVID-19?

One sign that does seem more specific to COVID-19 is the sudden loss of smell or taste.

Initially underappreciated as a symptom, the sudden loss of smell or taste is now thought to be a common symptom of COVID-19 – and may be the first symptom in up to 25 percent of COVID-19 cases, according to one review.

“That would be one symptom that I would be particularly concerned about for COVID-19,” said Fahimi.

Another sign that could point to COVID-19 is low oxygen saturation, which can cause shortness of breath, although it often goes unnoticed without a pulse oximetry measurement.

Chin-Hong noted that some symptoms in isolation are less likely to be COVID-19 – allergy-like symptoms, such as runny nose and water eyes, or diarrhea by itself – because these usually occur in conjunction with other symptoms of the disease.

Children under 10 are less likely to be infected with and show symptoms of COVID-19, so if they have flu-like symptoms, they are more likely to have the flu, he added.

But these are only generalizations. The variable nature of COVID-19 means the only way to know whether you are infected is to get a COVID-19 test.

“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 you do if you feel sick?

If you feel unwell with any of these symptoms, the best thing to do is to self-quarantine at home – including avoiding contact with other household members – and to get a COVID-19 test. Many testing sites take appointments without a referral from a doctor.

“We have to take an abundance of caution and try to catch all of these cases where we can,” said Fahimi.

Jahan Fahimi portrait

Jahan Fahimi, MD

The benefit of knowing you have COVID-19 is that you can take precautions to prevent spreading the disease, contact tracers can alert people you have been around recently, and doctors can follow up in case your condition worsens.

“We may be able to preemptively get someone into care a little bit sooner, which could impact their disease trajectory,” said Fahimi. 

Flu tests are also widely available but generally require a visit to the doctor’s office or urgent care. When diagnosed early, the flu can be treated with the antiviral Tamiflu.

Even if you test negative for both COVID-19 and the flu, it’s best to self-quarantine until three days after your symptoms disappear, said Fahimi. There’s a small chance that a negative test result could be wrong. Moreover, many other respiratory illnesses, from the common cold to croup, circulate during the flu season and spreading them would also spread uncertainty.

“Everyone’s focused on the flu and COVID-19, but there’s a whole host of other stuff going around, which can present very similarly,” said Chin-Hong. “So it’s going to be a confusing season and there’s going to be a whole lot of testing.”

When should people seek medical attention right away?

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

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

Do COVID-19 and the flu spread differently?

Peter Chin-Hong portrait

Peter Chin-Hong, MD

While both flu and COVID-19 are spread mainly by respiratory droplets launched through coughing and sneezing, COVID-19 can also transmit more stealthily, said Chin-Hong. One reason is that COVID-19 can also spread through aerosols, smaller droplets that can linger in the air and may be emitted through talking or singing.

Another reason is that people with COVID-19 can transmit the virus up to two days before they show symptoms or even if they never develop symptoms. About 45 percent of COVID-19 infections are asymptomatic. In contrast, said Chin-Hong, most people are well aware when they have the flu.

“That’s why COVID-19 is so insidious – because people don’t even know they are infected and they can spread it,” he said.

Will we have enough tests?

The most accurate tests for COVID-19 and the flu are both PCR tests that require some of the same reagents, or chemicals to process the tests, raising concerns that surges in both infections this winter could strain testing capacity.

“I think we’ve done a much better job of sorting out the supply chain that’s necessary to support a broader amount of testing,” said Fahimi. “So I’m hopeful that we’re not going to run out of tests.”

You can help prevent a shortage of tests, said Chin-Hong, by getting a flu shot. Although flu shots won’t prevent all cases of the flu, they will reduce the number of people with conflicting symptoms who may need tests, “and take a segment of the population out of that equation.”

The best way to avoid the “what do I have” conundrum this winter is to get the flu vaccine, and keep wearing masks, washing hands and maintaining social distance.

“We want to decrease the amount of people even 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 be in this position.”

This article is for educational purposes only and is not intended to replace the advice of your doctor or other health care provider. We encourage you to discuss any questions or concerns you may have with your provider.