Why get a flu shot if it might not work?
That question bubbles up every year as influenza sweeps the nation. Over the past 10 years, the vaccine at its best was 60 percent effective, according to data from the Centers for Disease Control and Prevention. At worst, it was about 20 percent effective.
This year, the H3N2 flu has been the main culprit, and the vaccine has struggled to keep up with changes in the virus. The CDC estimates it may be about 30 percent effective against H3N2.
"There's a lot of good methodology, but it's not foolproof, so some years are better than others," said John Shiver, global head of research and development at Sanofi Pasteur, which produces the bulk of flu vaccines in the U.S. "This year is one of those years where H3N2 is a bit divergent from what's in the vaccine, so the vaccine is appearing to be less effective overall."