ATLANTA (AP) -- Health officials say this year's flu season has shaped up to be the worst in four years, partly because the vaccine didn't work well against the viruses that made most people sick.
The 2007-2008 season started slowly, peaked in mid-February and seems to be declining, although cases are still being reported, according to the Centers for Disease Control and Prevention.
Based on adult deaths from flu and pneumonia, this season is the worst since 2003-2004. That was another time when the vaccine did not include the exact flu strain responsible for most illnesses.
Each year, health officials formulate a vaccine against three viruses they think will be circulating. They guess well most of the time, and the vaccine is often between 70 and 90 percent effective.
But this year, two of the three strains were not good matches and the vaccine was only 44 percent effective, according to a study.
Copyright 2008 by The Associated Press. All Rights Reserved.