ATLANTA - 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, health officials said yesterday.
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 - another time when the vaccine did not include the exact flu strain responsible for most illnesses.
Each year, health officials - making essentially an educated guess - formulate a vaccine against three viruses they think will be circulating.
They guess well most of the time, and the vaccine is often 70 percent to 90 percent effective.
But this year, two of the three strains were not good matches and the vaccine was 44 percent effective, according to a study done in Marshfield, Wis.
The CDC compares flu seasons by looking at adult deaths from the flu or pneumonia in 122 cities. This year, those deaths peaked at 9 percent of all reported deaths in early March, and remained above an epidemic threshold for 13 consecutive weeks. In 2003-2004, they peaked at more than 10 percent of all deaths, and surpassed the epidemic threshold for nine weeks.
"Our season is not quite as high but is lasting a little longer," said Dr. Dan Jernigan, deputy director of the CDC's influenza division.
Pediatric deaths are another way flu seasons are compared. This year, 66 children have died, including 46 who were not vaccinated.
In 2003-2004, 153 children died.
The CDC started working this year with the Marshfield Clinic in central Wisconsin to get a better gauge of vaccine effectiveness while a flu season was in progress.
Almost the entire population gets healthcare at the clinic, which has complete vaccination and electronic medical records.