The flu vaccine is more effective than expected, federal health officials said on Thursday at a special news conference held to discuss the dangerous flu season, which is expected to kill more than 50,000 Americans.

This year’s vaccine is about 25 percent effective against the H3N2 strain of flu that is causing most illnesses and deaths, said Dr. Anne Schuchat, acting director of the Centers for Disease Control and Prevention.

In a bigger surprise, the vaccine is about 51 percent effective in children, according to the C.D.C.’s preliminary analysis.

In Australia, the same vaccine was rated about 10 percent effective overall against H3N2, and a recent Canadian analysis found it to be about 17 percent effective there. (The C.D.C.’s final analysis will not be ready until the flu season ends in late spring.)