The flu vaccine barely worked in people over the age of 65 this year.
The Centers for Disease Control and Prevention released its report on Thursday.
The CDC report shows flu shots reduced the chances of illness by just 9 percent in people over 65. The vaccine was 58 percent protective in children, 46 percent effective in young adults and 50 percent effective in people aged 50 to 64.
CDC spokesman Tom Skinner says, "The flu season got off to an early start. It was a worse than average season but it was particularly severe in the elderly."
Health officials say they don't fully understand why the flu vaccine works so much more poorly in the elderly than it does in younger people. Some studies suggest people's immune response decreases as they get older.
Meanwhile, the CDC also says 64 children so far have died this year from the flu in the U.S.