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The flu vaccine is even less effective than health officials thought against the strain of influenza making most people sick this season. Getting the vaccine has reduced the risk of illness from the H3N2 strain of flu by only 18%, a new report says. The report comes from the U.S. Centers for Disease Control and Prevention (CDC). Officials said earlier in the season that the vaccine was about 23% effective. Among children, the vaccine offers even less protection, about 15%, the CDC said. The nasal spray vaccine may not protect children at all. One particular strain of H3N2 is causing most cases of flu in the United States so far this season. This strain was first seen in March 2014, the CDC said. That was too late to be included in the vaccine for this year. In most years, the flu vaccine is 50% to 70% effective. HealthDay News wrote about the CDC report February 26.