This season’s flu vaccine is 23 percent effective, according to the Centers for Disease Control and Prevention. Its effectiveness was reduced this year because between the time the components of the vaccine were determined and the fall when the vaccine was available the most prevalent influenza virus “drifted” or mutated.

“This estimate is relatively low compared with previous seasons when circulating viruses and vaccine viruses were well-matched,” CDC said in a Jan. 16 report on the vaccine effectiveness. Since CDC began conducting annual flu vaccine effectiveness studies in 2004-2005, the estimates for each season have ranged from 10 to 60 percent effectiveness in preventing medical visits associated with seasonal influenza illness, according to the press release.

In a sample of people tested in December, the proportion vaccinated varied from 46 to 66 percent across the states.

Overall, about half the people vaccinated still contracted the flu compared to 56 percent of the individuals who were not vaccinated. Another factor that influences how well the flu vaccine works is the age and health of the person being vaccinated.

In general, the flu vaccine works best in young, healthy people and is less effective in people 65 and older, and these results reflect that trend. The vaccine was most effective for Individuals younger than 17. For them it was 26 percent effective compared to 12 percent for individuals between 18 and 49, and 14 percent effective for people older than 50.

Nevertheless, CDC still recommends vaccination, especially for individuals who are at most risk for the flu and complications. This flu season probably has another six or seven weeks of duration, especially in California, where it has not yet been very intense.