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.