NASHVILLE, Tenn. -- A potent flu strain is infecting people in large numbers, and it’s not an exact match to current vaccines, according to doctors.

This year’s vaccine protects against three flu strains, but the Brisbane strain, a potent type-A strain, is infecting people in large numbers. Infections from the unexpected strain have been booming, and 44 states, including Tennessee, have reported widespread flu activity.

Metro Health Department Director Dr. Bill Paul said it’s a guessing game as to what flu strain will mutate from year to year.

“And they give it their best guess, but they have to commit to what’s going to be in the vaccine many months before the flu season, because they have to have time to make a safe vaccine. Well, sometimes a new flu strain will show up after that decision,” said Paul.

Paul said if someone does come down with the Brisbane strain and they got a flu shot, it will be a milder form than someone who didn’t get a shot.

Health professionals said residents should always get a flu shot and that February is usually the peak season for getting the flu.

The Centers for Disease Control and Prevention said the current vaccine is a good match for only about 40 percent of the spreading flu viruses.