That's one of the worst performances since the government started tracking how well vaccines work a decade ago. The best flu vaccines were 50 to 60 per cent effective.
But this year, the vaccine doesn't include the specific virus that is making most people sick. Early in the season, officials warned the vaccine probably would not work very well.
The study results were released Thursday by the Centers for Disease Control and Prevention. They studied about 2,300 people in five states who were sick.
CDC report: http://www.cdc.gov/mmwr