HOUSTON (KIAH) – October often marks the onset of flu season in the U.S. So, as cases begin to pick up, how do you know if you have COVID-19 or the flu?

The short answer: it can be hard to tell without testing. That’s because symptoms from both viruses can mimic one another, including headache, fever, congestion, and cough.

But there is one important clue a primary care physician at Baylor Scott and White shared with us.

“One difference that we do see, though, is the onset of symptoms. With the flu, usually within two to three days of coming into contact with someone with the flu, you will start to get these symptoms. Whereas with covid, it can be anywhere from a couple of days to over a week out from when you’ve come in contact with someone who has covid,” said Dr. Carol Nwelue, Baylor Scott & White Health.

According to data from the Centers for Disease Control and Prevention, this year’s flu shot is a good match for the circulating variants.

There’s also the updated COVID-19 vaccine that has become available.