Julie Gerberding, director of the Centers for Disease Control, blamed the shortfall on production problems at the vaccine's maker, the British pharmaceutical company Chiron, the Chicago Tribune reported. She said another 10 million doses should be shipped in the next month.
"We're sorry we have another frustrating year for some people," Gerberding said.
The newspaper said some experts believe demand is up sharply this year. Recent publicity about avian influenza's spread through Asia to Europe and the possibility of a deadly global pandemic if the virus mutates has raised public consciousness and fears about flu.
Gerberding said 71 million doses of vaccine have already been distributed.