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Influenza is killing more young and middle-aged adults this year than usual, in part because they’re less likely to be vaccinated, federal health officials said Thursday.

More than 60 percent of those killed or put into the hospital by flu so far this season have been aged 18 to 64, the Centers for Disease Control and Prevention says. And 50 children have died of flu so far.

The good news is the vaccine is pretty effective for a flu vaccine, with a 61 percent effectiveness rate, CDC and other experts found.

“If you are vaccinated you are quite likely to be protected form the flu viruses that are circulating this season,” the CDC’s Dr. Anne Schuchat told reporters on a conference call.

“The season is not over and things could change,” she added. “There’s still a lot of influenza circulating.” It’s not too late to get the vaccine, Schuchat said.

Flu usually hits the very young and the very old the hardest. Depending on the season, it kills anywhere between 4,000 and 50,000 people a year in the United States.

"Younger people may feel that influenza is not a threat to them, but this season underscores that flu can be a serious disease for anyone."

This year, the main circulating strain is H1N1 flu, which first showed up in 2009 when it caused a new pandemic. And like in 2009, it’s hitting people under 65 the hardest.

“They’re less likely to have natural immunity and they’re less likely to have been vaccinated,” Schuchat said.

CDC officials believe that people over about 60 are likely to have been infected with a distant cousin of H1N1 in the past, so they may have natural immunity. And children are more likely either to have had H1N1, or to have been vaccinated against it, or both.

Last flu season, people 18 to 64 years old accounted for only 35 percent of hospitalizations, the CDC report notes.

"Younger people may feel that influenza is not a threat to them, but this season underscores that flu can be a serious disease for anyone," Frieden said. "It's important that everyone get vaccinated.”

Yet only 34 percent of adults 18 to 64 have been vaccinated this season, and only 41 percent of children and 41 percent of pregnant women, who are very susceptible. More than 60 percent of people over 65 got vaccinated this year, CDC found.

While even perfectly healthy people can get severely ill from flu, most of those who did get sick enough to be hospitalized had some underlying condition such as heart disease or lung disease. Obesity is an underlying condition, too, that makes people more likely to get very ill, and two-thirds of U.S. adults are overweight or obese.

CDC cannot keep a precise count of the number of adults hospitalized or killed by influenza, but does keep track of children who die. Computer models track adult deaths and hospitalizations.

Officials said it’s frustrating that more Americans don’t get vaccinated, and they are trying to make it easier to get flu vaccines — at work, for example, and in stores. And they reminded doctors to treat seriously ill people with antiviral drugs such as Tamiflu right away if they even suspect flu.