JACKSONVILLE, Fla. -- Fighting off the flu has become tougher than usual at this time of year even for people who have had a flu shot.

According to federal health officials, the flu season is getting worse because the vaccine that's out there doesn't protect against more than half of this year's viruses.

"The flu vaccine may only be 40 percent effective," said Dr. Harold Laski.

This year, the flu shot may not work as well as it did in years past.

"I think we've had like 19 seasons or so since we've had the flu shot, and most of them have been pretty good guesses. Most of them have were about 70-90 percent effective, but that doesn't mean every year. We've had years where it's been 50-60, and this year it just happens to be one on the curve where it's 40 percent," Laski said.

The doctor said he was still going to give the flu shot this year, but that he was going to tell his patients about it being less effective than in past years.

"There's no problem with the vaccine. It's not causing deaths more than any other vaccines in the past. It's not a more dangerous vaccine. It's just no working as well," Laski said.

The problem is the people making the flu vaccine every year simply have to guess which strain will get people sick. They really never know 100 percent what that's going to be.

This year, they may have guessed wrong.

"It's a science in order to guess. It's not easy in the first place, but it's best we can do right now," Laski said.

Doctors said one of the best ways to prevent the flu is by washing your hands.