If you're younger than age 50 and have had a flu vaccine in the past, U.S. researchers say a half-dose flu shot may be enough to protect you from the flu. In a recent study at Walter Reed Army Hospital, investigators gave 554 healthy people a full-dose flu shot and 556 healthy people a half-dose flu shot. All of the participants had received a flu shot in the past three years. After 21 days, the researchers found little difference in the antibody responses in the two groups. Experts say this...