ATLANTA - The current 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 1997-1998, when the vaccine didn't work at all against the circulating virus, according to the Centers for Disease Control and Prevention.

The 2007-2008 season started slowly, peaked in mid-February and seems to be declining, although cases are still being reported, CDC officials said.

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 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 done in Marshfield, Wis. That seemed to match the experience in other parts of the country.

"We've had a pretty heavy season, both adult and pediatric. And there were a good number of cases — more than usual — who had received a vaccination," said Dr. Niranjan Bhat, a children's infectious disease specialist at Johns Hopkins Hospital in Baltimore.

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. 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. So far this season, 66 children died, including 46 who were not vaccinated. In 2003-2004, 153 children died.

Each year, the flu results in 200,000 hospitalizations and 36,000 deaths, according to official estimates. The elderly, young children and people with chronic illnesses are considered at greatest risk.

The CDC started working 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 in the Marshfield area — about 50,000 people — gets health care at clinic offices, which has complete vaccination and electronic medical records.

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, contributing to this season's severity, CDC officials said.

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

Jernigan acknowledged that some people may lose faith in the flu vaccine and skip it next year. But he noted even this year's mismatched vaccine still offered 44 percent protection overall and likely reduced the severity of illness in those who got the flu.

The Marshfield study and a flu season update are being published this week in a CDC publication, Morbidity and Mortality Weekly Report.

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