WASHINGTON — Think the flu’s only a big threat to kids and seniors? Influenza hospitalized a surprisingly high number of young and middle-aged adults last winter — and this time around the government wants more of them vaccinated.

Fewer than half of Americans get a flu vaccine every year. Vaccination rates are highest for children under 5 — 70 percent — and for seniors — 65 percent, the Centers for Disease Control and Prevention said.

But just a third of healthy adults ages 18 to 64 got vaccinated last year, the CDC said. And last winter, that age group had its highest rate of flu-related hospitalizations since the 2009 pandemic.

It’s not clear why that age shuns the vaccine, although some studies suggest they don’t think they’re at risk.

“Vaccination is the single most important step everyone 6 months of age and older can take to protect themselves and their families against influenza,” said CDC Director Tom Frieden.