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Flu season kicks in, likely to affect younger adults, CDC says

IMAGE DISTRIBUTED FOR NATIONAL COUNCIL ON AGING AND SANOFI PASTEUR - Pharmacist Jason To gives a flu shot to Joao Ferreira during the Flu + You event ...
IMAGE DISTRIBUTED FOR NATIONAL COUNCIL ON AGING AND SANOFI PASTEUR - Pharmacist Jason To gives a flu shot to Joao Ferreira during the Flu + You event at Montclair Recreation Center, sponsored by the National Council on Aging and Sanofi Pasteur on Tuesday, Oct. 22, 2013, in Denver. (Jack Dempsey / AP Images for National Council on Aging and Sanofi Pasteur)Jack Dempsey / AP
Pharmacist Jason To gives a flu shot to Joao Ferreira in October. The dominant strain of flu this year is more likely to affect younger adults than the elderly, federal health officials say.Jack Dempsey / AP

The new flu season is ramping up across the U.S. with growing reports of illness — particularly in the south — chiefly caused by the H1N1 bug that is more likely to sicken younger adults than the elderly, health officials said Friday. 

Flu activity is increasing nationwide and is already high in six states: Texas, Louisiana, Mississippi, Alabama, Oklahoma and Missouri, officials with the Centers for Disease Control and Prevention said. 

For the first time since the 2009 influenza pandemic, H1N1 is the dominant circulating flu strain early in the season, CDC officials said. While most flu strains predominantly sicken the elderly and those with existing health problems, that strain mostly sickens younger adults, those ages 18 to 49, and middle-aged folks, ages 49 to 64.

CDC officials warned earlier this week that they have already received "a number" of reports of serious respiratory illness and death in young and middle-aged adults, including many infected with H1N1 flu. In Texas, where flu is widespread in all areas, a 17-year-old with underlying health problems died, health officials said. 

"It's a reminder that flu can be a serious disease," said Dr. Joseph Bresee, a CDC flu expert. 

So far, there have been no significant changes in the H1N1 flu viruses to suggest they're spreading more easily or have become more virulent, but CDC officials said they're monitoring for any signs. 

It's still too soon to tell how serious this year's flu season will be, or how well this year's vaccine matches the strains that are actually going around, Bresee said. But, he added, a flu shot is still the best way to avoid illness. 

CDC recommends that everyone older than 6 months get a flu shot every year. Federal officials reported earlier this month that flu vaccinations kept nearly 80,000 people out of the hospital last year and prevented 6.6 million cases of the flu. 

On average, CDC says 200,000 people are hospitalized with flu each season — ranging from 158,000 people hospitalized in 1990-1991 to 431,000 in 1997-1998. Flu vaccine also may prevent other conditions, such as heart disease, studies show. Flu season typically peaks in January and February. 

Still, many Americans, particularly young adults, tend not to get vaccinated. 

JoNel Aleccia is a senior health reporter with NBC News. Reach her on Twitter at @JoNel_Aleccia or send her an email.