Vaccine blamed for severe flu season

ATLANTA — The 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 Thursday.

This season's vaccine was the worst match since 1997-98, when the vaccine didn't work at all against the circulating virus, according to the Centers for Disease Control and Prevention (CDC).

The 2007-08 season started slowly, peaked in mid-February and seems to be declining, CDC officials said.

Based on adult deaths from flu and pneumonia, this season is the worst since 2003-04, another time the vaccine did not include the exact flu strain responsible for most illnesses.

Each year, health officials — making 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 between 70 percent 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 done in Marshfield, Wis. That seemed to match the experience in other parts of the country.

The CDC compares flu season 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.

Pediatric deaths are another way flu seasons are compared. So far this season, 66 children have died, including 46 who were not vaccinated. In 2003-04, 153 children died.

This year, most of the illness has been due to Type A H3N2 Brisbane strain, which was not in the vaccine. That strain tends to cause more hospitalizations and deaths, CDC officials said.

Type B Florida strain, also absent from this year's vaccine, also has caused illness. Marshfield data showed that the vaccine was completely ineffective against the Type B virus and was 58 percent effective against the Brisbane virus.