Flu Shot Just 23% Effective That's one of the worst marks in last decade By Newser Editors and Wire Services Posted Jan 15, 2015 7:17 PM CST 29 comments Comments This 2011 image provided by the Centers for Disease Control and Prevention shows an H3N2 influenza virus. (Uncredited) (Newser) – As predicted, this year's flu vaccine is doing a pretty crummy job. It's only 23% effective, primarily because it doesn't include the bug that's making most people sick, according to a government study out today. That's one of the worst performances in the last decade, since US health officials started routinely tracking how well vaccines work. In the best flu seasons, the vaccines were 50% to 60% effective. "This is an uncommon year," says Dr. Alicia Fry, a flu vaccine expert at the CDC who was involved in the study. In December, CDC officials warned the vaccine probably wouldn't work very well because it isn't well matched to a strain that's been spreading widely. Each year, the flu vaccine is reformulated based on experts' best guess at which three or four strains will be the biggest problem. Those decisions are usually made in February, months before the flu season, to give companies that make flu shots and nasal spray vaccine enough time to make enough doses. But this year's formula didn't include the strain of H3N2 virus that has caused about two-thirds of illnesses this winter.