 -- Fifteen children have died from complications of the flu so far this season, the Centers for Disease Control and Prevention noted, as it officially declared the illness an epidemic.

The number of states reporting a high amount of “influenza-like” illness activity has increased from 13 to 22 since last week’s report from the agency, with outbreaks in every region of the country.

Hospitalizations also climbed this week with seniors and kids younger than 4 accounting for the highest rate of hospitalizations.

At least six Tennessee children have died from the flu this year, the state's Department of Health reported. Tennessee is under the widespread outbreak category, as of Monday, according to the CDC. So far, East Tennessee Children’s Hospital has seen 442 children with the flu just this month.

While this year’s strain of the virus is especially severe, ABC News chief health and medical editor Dr. Richard Besser said, flu can always be deadly for children, the elderly and anyone with a compromised immune system.

“Every year about a hundred children die from the flu,” he said today on “Good Morning America.”

About 90 percent of flu cases so far this year have been the H3N2 subtype, the CDC reported.

Flu strains are named for molecule types surrounding the outside of the virus particle, said Dr. Pritish Tosh, an infectious diseases physician with the Mayo Clinic and a member of the Mayo vaccine research group.

There are 17 different kinds of hemagglutinin, or H particles, which allow the virus to bind to cells, and nine different kinds of neuraminidase, or N particles, that allow the virus to spread the infection throughout the body, he said.

H3 subtypes tend to lead to the largest number of hospitalizations and deaths, Tosh said.

The hardest hit states are in the south, Midwest and western parts of the country, though Patsy Stinchfield, the director of infection prevention and control with Children's Hospitals and Clinics of Minnesota, said it’s unclear why.

“The one thing we know about influenza is that it is unpredictable,” Stinchfield said. “We don't fully understand why but low vaccination rates may have something to do with areas getting hit harder.”

Besser said for the past four years flu season has been hitting earlier and earlier and that is a worrying trend.

“It seems to be peaking at the end of December and it used to be it did not peak until February or March,” he said.

Although the current vaccine does not seem to be a good match for this year’s strain of the virus, Besser said, it still makes sense to get a flu shot, especially if you are in a high-risk group.

“There may be some level of cross-protection,” he added,” but we won’t know until March or so until we look back.”