The proportion of all deaths in the United States last week caused by pneumonia and influenza reached 7.3 percent, above the epidemic threshold of 7.2 percent, the U.S. Centers for Disease Control and Prevention said on Friday.
Only the Southwest and California had "normal" flu activity last week, the agency said. The other nine regions had "elevated" activity, confirming that seasonal flu has spread across the country and reached high levels several weeks before the usual time of late January or February.
The vaccine against the flu strain that was forecast to predominate this year, called H3N2, is 62 percent effective, scientists reported on Friday in the CDC's weekly publication.
That is considered "moderate" effectiveness and means that almost four in 10 people who receive the vaccine and are exposed to the virus will nevertheless become infected. But the vaccine can reduce the severity of the illness, preventing pneumonia and other life-threatening results of flu.