Why this year's flu vaccine isn't as effective?

(CNN) -- This year's influenza virus is particularly difficult to fight.

New strains pop up as the season wears on, making it tough to mass-produce an effective vaccine.

"Every year it's a bit of a guessing game to decide what strain of the virus gets put into the vaccine," says Dr. Mark Rupp with the University of Nebraska Medical Center's Division of Infectious Diseases.

Before flu season begins, researchers study the influenza strains circulating in the southern hemisphere, allowing them to identify strains that could show up in North America. But how we manufacture the vaccine might be working against us.

The FDA requires vaccine viruses be grown in chicken eggs. But some strains -- like this year's H3N2 -- don't grow well in eggs.

"The process for the virus to be adapted into the chicken eggs actually caused the virus to change a little bit and because of that change, the vaccine doesn't fully recognize the wild-type strain that's in circulation," Rupp says.

Scientists want to create a flu vaccine that lasts a lifetime.

"We need to discover or to define a better universal target so that we don't have to change the vaccine from year to year, and this target has to allow for long-lasting immunity."

Until then, doctor's recommend you to get the flu shot every year. Even if you get the flu, it can help your body fight back--reducing the amount of time you're sick.