ATLANTA (AP) -- Flu season seems to be winding down, and it's been an odd one.
It hasn't been as bad as last year and the vaccine worked a little better. And it has been a fairly mild one for the elderly - traditionally the most vulnerable group.
But it's been a different story for young and middle-age adults, who have been hit harder than expected because of a surge in swine flu.
Most flu seasons, only one-third of the people who land in the hospital with the flu are adults ages 18-64. This winter, they have accounted for two-thirds, most of them adults who were obese or had another ailment.
The numbers "are painful reminders that flu can be serious for anyone, not just infants and the very old," said Dr. Tom Frieden, director of the Centers for Disease Control and Prevention.
The CDC released new information Thursday about the flu season, which started around mid-December, a bit earlier than usual, and apparently peaked by mid-January. Last winter, it started even earlier.
For weeks, flu has been waning in some parts of the country, although health officials say there's still a lot of flu virus going around and flu season is capable of getting a second wind.
One way the CDC measures the flu's severity is by rates of hospitalizations for flu and its complications. Overall, they have been only half what they were last winter despite the higher rates in younger adults.
Health officials say that's because the flu strain that is making most people sick this winter is swine flu, or H1N1. That kind first showed up in 2009 and caused a global pandemic that was particularly dangerous to younger adults.
Since then, the virus has been around each winter but mostly has been a backstage presence. Experts say the virus hasn't mutated. It's simply encountering a lot of younger adults who never were infected before and haven't been vaccinated, said CDC flu expert Dr. Joseph Bresee.
The annual flu vaccine is modified each year, and swine flu is now included in the mix of seasonal strains. Working-age adults have the lowest vaccination rates, CDC officials noted.
Among infectious diseases, flu is considered one of the nation's leading killers. On average, about 24,000 Americans die each flu season, according to the CDC. The agency doesn't keep an exact count of flu deaths. But health officials estimates that 60 percent of flu deaths this season have been in people ages 25 to 64, similar to the numbers from 2009-2010.
The government also released data on this year's flu vaccine. A flu vaccine that's 60 to 70 percent effective in the U.S. is considered pretty good and this year's falls in that range.
Overall, it was a moderate 61 percent. That means those who got a shot have a 61 percent lower chance of winding up at the doctor with the flu. Last year, it was 56 percent effective.
Those numbers reflect how good the vaccine was against swine flu. There wasn't enough data to really gauge how well it worked in the elderly against a strain that's more dangerous to older adults. Last season, the vaccine was only 9 percent effective in people 65 and older against that H3N2 strain - one reason last flu season was unusually severe.