Hardest hit this U.S. flu season are adults, from younger to middle-aged, in part because fewer have been vaccinated, a federal health agency said on Thursday as the season enters its final weeks.

An estimated 60 percent of those who have died from influenza this season were 24 to 64 years old, compared with 18 percent last season, the Centers for Disease Control and Prevention said.

People ages 18 to 64 have accounted for 61 percent of all hospitalizations due to the flu, up from about 35 percent in the three prior seasons, the CDC said.

This season’s dominant flu strain is the H1N1 virus, the same one that struck in 2009, with high rates of hospitalization and death among young adults and the middle aged, the CDC said.

The current flu season has not been as severe as 2009 and 2010, the CDC said, but officials noted it was too early to say how it will compare with last year’s.

Flu activity varies across the country, with some states still experiencing high levels while others see decreases.

The latest statistics offer “a sad and painful reminder that flu can be serious for anyone, not just the very young and old,” said CDC Director Tom Frieden.

One reason younger people have been more heavily affected by the flu this year is that their vaccination rates are lower than older people’s, Frieden said. The CDC recommends vaccinations for anyone over 6 months.

Older patients also may have stronger immunity to H1N1 due to past exposure to the virus, he said.

The CDC bases its flu estimates on a sample of data from 122 cities. It does not yet have exact numbers of how many people nationwide have died or been hospitalized this season, which began last fall and is expected to last a few more weeks.

Health officials say they are encouraged because this year’s vaccine has reduced a patient’s chances of having to go to the doctor with the flu by 61 percent, compared with 52 percent last year.

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By David Beasley