(CBS NEWS) This year's flu season has hit younger and middle-aged adults harder than in past years, government health officials warned Thursday. In previous flu seasons, young children and seniors aged 65 and older were among those most likely to be hospitalized or die from flu.

But according to the Centers for Disease Control and Prevention, adults between ages 18 and 64 represented about 61 percent of all flu-related hospitalizations this year, with influenza deaths following the same pattern.

This age group accounted for 35 percent of all flu hospitalizations in the previous three flu seasons, health officials report in the Feb. 20 issue ofMorbidity and Mortality Weekly Report, the CDC's medical journal.

The age group also tends to have the lowest vaccination rates, Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, told reporters during a press conference.

"Flu hospitalizations and deaths in people -- younger and middle-aged adults -- is a sad and difficult reminder that flu can be serious for anyone, not just the very young and old; and that everyone should be vaccinated," said Dr. Tom Frieden, director of the CDC. "The good news is that this season's vaccine is doing its job, protecting people across all age groups."

One of the predominant disease strains circulating this year was

H1N1, or "swine flu," which was responsible for a 2009 pandemic

that killed up to 200,000 people worldwide.

H1N1 affects healthy younger peoplein addition to traditionally higher-risk groups,Dr. Susan Rehm, medical director of the National Foundation for Infectious Diseases (NFID), told CBS News in January.

This year saw several high-profile reports of previously healthy young people dying from flu in areas includingTexasand theSan Francisco Bay Area.

During the 2009 pandemic, 18- to 64-year-olds accounted for about 56 percent of flu hospitalizations.

This year's flu vaccine was designed to protect against 2009 H1N1.

According to November estimates, about 60 percent of seniors received a flu vaccine and 50 percent of young children got a flu shot. But only about one-third of people ages 18 to 64 got vaccinated.

The CDC did point out that some vaccinated people still develop severe cases of flu.

"It's also important to remember that some people who get vaccinated may still get sick, and we need to use our second line of defense against flu: antiviral drugs," said Frieden. He added, "We are committed to the development of better flu vaccines, but existing flu vaccines are the best preventive tool available now."

While hospitalization rates were still highest among people 65 and older -- about 60 seniors per 100,000 flu cases hospitalized -- adults 50 to 64 years old now have the second-highest hospitalization rate (38.7 per 100,000), followed by children younger than 4 (35.9 per 100,000).

Most of those hospitalized had preexisting medical conditions, with obesity one of the main culprits. About 15 percent of adults who werehospitalizeddid not have a medical condition, said Schuchat.

Fifty children have died from flu this year, compared to 169 in last year's flu season. The CDC only tracks pediatric death rates, and its conclusion that adults 18 to 64 were dying in higher rates this year factored in hospitalization rates and anecdotal reports.

As of Feb. 8, themost recent data collected by the CDC, six states continued to show high flu activity: Arkansas, Connecticut, Kansas, New York, Oklahoma, and Texas.

The flu vaccine is recommended for everyone aged 6 months and older, including pregnant women. Children younger than 9 who have yet to get a flu shot may need two doses to build up immunity -- parents can check with their doctor. Adults who were vaccinated in the early months of the flu season should still be protected.

"I want to remind you the season is not over, and things can change," said Schuchat. "Vaccinated people were substantially better off than people who did not get vaccinated. If you haven't been vaccinated yet, it's not too late to get benefits," she said.

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