FRIDAY, Jan. 10, 2014 (HealthDay News) -- Flu season continues to tighten its grip on the United States, with 35 states now experiencing widespread influenza activity, federal officials reported Friday.

The flu seems to be hitting young and middle-aged adults the hardest this year, rather than seniors or children, said Dr. Michael Jhung, a medical officer in the U.S. Centers for Disease Control and Prevention's Influenza Division.

About 61 percent of the 2,622 flu-related hospitalizations this season have been for people between 18 and 64 years old, Jhung said. Typically, seniors 65 and older account for more than half of flu hospitalizations during a season.

This could be because the H1N1 strain of flu is the most prevalent this season, and most younger adults don't get a flu shot, said Dr. Daniel Spogen, chair of the department of family and community medicine at the University of Nevada School of Medicine.

"The ones that tend to be sick this season are young and otherwise healthy adults," added Spogen, who's also a board member of the American Academy of Family Physicians. "If you take a look at the data, the people who are getting sick enough to be hospitalized are the ones who didn't get their flu shot."

Across all age ranges, about 40 percent to 45 percent of people get an annual flu shot, Jhung said. But only 31 percent of people 18 to 49 years old usually receive a flu vaccination, he said.

Young and middle-aged adults also were disproportionately affected during the 2009 H1N1 flu pandemic, Jhung added.

Another reason for that trend: Until the 2009 pandemic, the H1N1 flu strain had been dormant for several decades, leaving younger adults with little immunity to the virus.

Still, the number of deaths related to either the flu or pneumonia this season remains slightly below epidemic levels, the CDC said.

An estimated 6.9 percent of all deaths in the United States this season have been caused by flu or pneumonia -- just under the epidemic threshold of 7.1 percent. Ten children have died from the flu so far this season, the CDC said.