Preliminary findings, published Thursday in the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report, were based on a survey of 2,697 children and adults enrolled in the U.S. Influenza Vaccine Effectiveness Network from Dec. 3, 2012, to Jan. 19, 2013.
Among the patients with influenza, 32 percent had gotten the 2012-13 seasonal influenza vaccine, compared with 50 percent of the influenza-negative controls. For people with medically attended acute respiratory illness, the overall vaccine effective against influenza A and B virus infections was 56 percent, the report said.
Among the 751 infections with influenza A viruses, 98 percent of the infections were caused by influenza A (H3N2) viruses.
The adjusted vaccine effectiveness for all ages against influenza A (H3N2) virus infection was 47 percent; 58 percent for persons age 6 months-17 years, 46 percent for persons ages 18-49, 50 percent for those ages 50-64 and 9 percent for persons age 65 and older.