THURSDAY, April 17 (HealthDay News) -- The current U.S. flu season has been the worst in four years, due, in part, to a vaccine that was not a good match for certain circulating strains of flu virus, U.S. health officials said Thursday.

For strains of influenza A (H3N2) -- the most prevalent virus during the 2007-08 season, the vaccine was 58 percent effective. But it was 100 percent ineffective against influenza B infections, leaving an overall vaccine success rate of about 44 percent, according to the U.S. Centers for Disease Control and Prevention.

"Most of the circulating influenza viruses this season have been less than optimally matched to the viruses in the vaccine," Dr. Dan Jernigan, deputy director of CDC's Influenza Division, said during a teleconference. "However, the vaccine did provide substantial protection against the predominant influenza virus circulating this season -- the H3N2 influenza A virus. Those people vaccinated were 58 percent less likely to have H3N2 infection than those not vaccinated."

The CDC researchers realized that two of the three circulating strains of flu this season did not match the strains contained in the vaccine, based on results of a study done in Marshfield, Wisc.

The CDC began working with the Marshfield Clinic in central Wisconsin to gauge the effectiveness of influenza vaccines during the flu season. Almost all people in Marshfield receive their health care from the clinic, according to a report in the April 18 issue of the CDC's Morbidity and Mortality Weekly Report.

This flu season, Type A H3N2 Brisbane strain has caused most of the illnesses but was not in the vaccine. The influenza A Florida strain, also not in the vaccine, has also caused sickness.

Jernigan noted that while this season's flu vaccine wasn't perfect, this year's results support getting vaccinated, even in years when the vaccine match is less than optimal.

"Although influenza A viruses have predominated this season, the most recently isolated viruses are influenza B viruses," Jernigan said. This year's vaccine is ineffective against influenza B viruses, he said.

Although the overall effectiveness of this year's vaccine was 44 percent, it has been higher in some years and lower in others, Jernigan said. "In the last 20 seasons, 16 have had good matches, and there have been four that were less than optimal matches," he said.

During the 1997-98 flu season, the vaccine's effectiveness was essentially zero, Jernigan said, adding that was the first year the Type A H3N2 influenza virus appeared.

In some years, the flu vaccine has had an effectiveness level of 70 percent, Jernigan said.

During the current flu season, the number of deaths peaked at 9.1 percent of all reported deaths, Jernigan said. "The number of deaths exceeded the epidemic threshold for 13 consecutive weeks," he said.

There were 66 flu-related deaths of children, 56 of whom weren't vaccinated, were improperly vaccinated or were too young to be vaccinated, Jernigan said.

During the 2003-04 season, deaths peaked at more than 10 percent and remained above the epidemic threshold for nine weeks, Jernigan said.

Each flu season, there are three different strains of influenza virus circulating. Different strains appear at different times, and different strains predominate, Jernigan explained. In addition, each of the three strains can respond differently to the flu vaccine.

It is this fluid flu picture that necessitates changing the vaccine from year to year, Jernigan said.

Since it requires almost a year to prepare the vaccine, decisions about the strains to include in the vaccine are often an educated guess.

But a study published in the April 17 issue of the journal Science could eliminate much of that guess work. Researchers reported that flu viruses originate in East Asia and Southeast Asia, and it takes about eight to nine months before these new viruses reach western Europe and North America.

Jernigan said the CDC is currently involved in promoting better surveillance of emerging flu viruses in East Asia and Southeast Asia. The hope is that these surveillance efforts will lead to more effective and better-matched vaccines.

According to the CDC, each year from 5 percent to 20 percent of the U.S. population gets the flu. More than 200,000 people are hospitalized from flu complications, and about 36,000 people die from the disease. Some people, such as older people, young children, and people with certain health conditions, are at high risk for serious flu complications.

More information

To learn more about the flu, visit the U.S. Centers for Disease Control and Prevention.