North Texas counties are showing a spike in cases that doesn't typically occur until after Christmas.
Tarrant County said it's already seeing widespread influenza activity, and Dallas County has had a dramatic spike in cases in the last 30 days.
In November, 217 confirmed flu cases were reported, up from just 24 cases in October.
Many patients with flu-like symptoms are showing up at hospital emergency rooms.
"We've probably been seeing somewhere between five, maybe five to 10 a day," said Dr. Mark Till of Texas Health Presbyterian Dallas.
Dr. Jane Sadler, of Baylor Medical Center of Garland, said she wonders if the flu vaccine covers all of the strains that are circulating. She has seen three patients in recent days who tested positive for influenza Type A even though they had a flu shot.
"I actually thought I had pneumonia, because everything was kind of in my chest and it hurt when I coughed," said Amy Bartis, one of the patients.
"I guess I was relieved that I didn't have pneumonia, but I was still shocked that I had the flu since I had a flu shot," she said.
Sadler said she was also surprised.
"Really, the flu shot this year offers great coverage for the flu," she said. "But she's not the only person that we've had with a positively test despite receiving a flu shot."
Doctors said people who have not had a flu shot need to get one immediately.
People who have a fever higher than 101.5 degrees, severe body aches and a sore throat or cough should have their symptoms quickly evaluated because antivirals such as Tamiflu work best if administered early.
NBC 5's Omar Villafranca contributed to this report.