Is it cold, flu or Covid? What to know about symptoms and testing

Early clues suggest that this flu season could be particularly brutal. And while national COVID rates currently appear to be low, researchers expect the virus to circulate more widely in the coming months as people gather for the holidays.

If you feel ill but your symptoms are only above the neck -- a stuffy nose, a bit of a sore throat -- you could just have a common cold, said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center.

But a hacking cough, muscle aches and full-body fatigue likely indicate a viral infection such as flu or COVID. Even infectious disease specialists struggle to tell which virus is which from symptoms alone.

The symptoms

Both flu and COVID can induce coughs, shortness of breath, fatigue, runny noses, head and body pains, sore throats and gastrointestinal issues such as stomach pain and diarrhea. The illnesses can also blunt people's sense of smell -- although with influenza, that typically happens because of nasal inflammation, while COVID can impact nerves that help people smell. Fewer people are losing their sense of smell now from COVID than did in the early days of the pandemic.

"I can't give you a clear indicator that if your nose turns green" it's one or the other, Schaffner said.

One key differentiator, though, is the onset of symptoms. COVID often comes on gradually: your head clogs with congestion one day, your throat aches the next. With flu, people frequently describe feeling like they've suddenly been "hit by a dump truck," said Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco.

Fevers can be a symptom of both infections, but your temperature tends to spike earlier during a bout of flu.

Testing is key

The only sure way to discern COVID from flu is to take a test.

Many at-home COVID tests are still available online and at drugstores. At-home tests that detect both COVID and the flu have recently become available, too, though they are typically slightly more expensive. Both kinds of tests often deliver results within around 15 minutes.

At-home COVID tests can give false negatives if you test too early during a bout of illness, because it takes time for enough viral load to build up in the body to trigger a positive result. That's why doctors say that if you test negative but still have symptoms you should take a second test at least 48 hours after your first.

It's particularly important to take a test if you are at high risk for getting seriously sick from either illness, so that you can consider proper treatment, which could include the antiviral drugs Paxlovid or Tamiflu. People who are 65 and older, pregnant women and people with underlying health conditions, such as asthma or kidney, blood or liver issues, are more likely to develop a serious illness.

And testing can just help you plan your life, said Dr. Preeti Malani, an infectious disease physician at University of Michigan Health. COVID and flu tend to last longer and be more severe than other run of the mill respiratory viruses, she said; knowing what you're sick with can help determine how long you expect to be out of work or school, or whether you may be sick for long enough to miss a holiday gathering.

Just as both illnesses look alike, you can prevent their spread in similar ways: frequent hand-washing, sticking to places with good ventilation, avoiding crowds, masking, staying home when you're sick and getting vaccinated.

Doctors stressed that it is not too late to get either a flu shot or an updated COVID vaccine.