Current Flu Season Worst in 4 Years
Thursday, April 17, 2008; 12:00 AM
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'sMorbidity 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.