April 17, 2008 -- This year's flu shot was only 44% effective, a new study suggests -- the least effective flu vaccine in a decade.
The findings come from a study of 616 Wisconsin residents who came down with flu-like illnesses during the peak of the flu season. Study findings appear in today's issue of the CDC's Morbidity and Mortality Weekly Report.
In good years, flu vaccines are 70% to 90% effective in preventing confirmed cases of flu bad enough to cause a person to seek medical attention. This year's vaccine appears to be the least effective since the 1997-1998 flu season, when the vaccine was about 50% effective.
Two of the 2007-2008 flu vaccine's three components didn't match most of the flu viruses circulating this flu season. This season's predominant flu bugs have been the mismatched type A H3N2 and type B strains.
The H3N2 component was only a partial mismatch, cutting the vaccine's effectiveness against type A flu to 58%. The type B flu component did not match at all -- and the vaccine seems to have had no effect against this bug.
Dan Jernigan, MD, MPH, deputy director of the CDC's influenza division, takes an optimistic, glass-half-full view of the study findings.
"While the vaccine's effectiveness against H3N2 is less than might be expected ... the evidence suggested that the vaccine provided substantial protection," Jernigan said at a CDC news conference. "The measurable effectiveness of the vaccine in this study suggests we continue to recommend vaccination even in years of mismatch."
That's true: The vaccine's 44% effectiveness is a lot better than the 0% effectiveness of no vaccination at all.
And that's good news for the record number of Americans who got their flu shots this year. Vaccine companies delivered 113 million doses of flu vaccine.
"That is more flu vaccine than ever distributed in the U.S. before -- about 10 million more doses than were distributed last season," Jeanne Santoli, MD, MPH, deputy director of the CDC's immunization services division, said at the news conference.
'Moderately Severe' Flu Season Still Simmering
For 13 weeks this season, flu deaths were above what CDC calls "the epidemic threshold." This means that flu deaths made up a larger-than-normal proportion of all deaths. At the peak of the season, in mid-February, flu deaths peaked at 9.1% of all U.S. deaths.
That makes this year's flu season similar to the 2003-2004 season, when flu deaths peaked at 10.4% of all deaths and exceeded the epidemic threshold for nine weeks.
The 2003-2004 season was officially labeled "moderately severe." There won't be an official designation for this flu season until it officially ends in May.
And flu season is not yet over. Six states -- Connecticut, Maine, Maryland, New York, Pennsylvania, and Vermont -- still had widespread flu outbreaks as of April 5.
Although the "Brisbane" H3N2 type A flu virus was the most common flu bug this year, Jernigan said, the type B virus is now the predominant strain currently going around.
Since the flu vaccine isn't effective against this bug -- and isn't totally effective against the Brisbane bug -- the CDC is recommending that people who come down with flu-like illnesses ask their doctors about the flu drugs Tamiflu and Relenza.