Every year, medical experts face the challenge of quickly developing a vaccine to match the current strain of flu, and last season's vaccine was effective only about half the time.
Scientists say that's typical in years when the shot and the virus aren't a perfect match. In 1997, for example, the vaccine was about 30 to 50 percent effective.
The Centers for Disease Control and Prevention says flu shots are still beneficial. When the vaccine more closely matches the type of flu circulating, it's up to 90 percent effective in healthy adults.
Last fall, the Fujian flu strain emerged in Asia too late to be included in the vaccine formula. But the CDC says despite the less-than-perfect match, the number of cases and deaths was fairly typical for a flu season.