U.S. health authorities said vaccine production for the H1N1 influenza virus is running behind by about 25 percent, or 10 million doses.

The federal Centers for Disease Control and Prevention had been expecting to have around 40 million doses of the vaccine for H1N1, also known as swine flu, prepared by the end of October, but current estimates suggest only 28 million to 30 million doses will be ready by that time, the Los Angeles Times reported Friday.

Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, said the delays are related to the method of creating antigen, the main component of the vaccine, from eggs. She said the process is not as effective with H1N1 as it is with the seasonal flu.

"Vaccine production is pretty complex, and it is taking a bit longer than we would hope," she said.

Schuchat said Friday officials have documented widespread H1N1 activity in 41 states, up from 37 states last week, and officials in the remaining states have reported regional or local cases of the virus. She said the level of influenza activity "is unprecedented for this time of year."

"That's very high at any time, but particularly in October," Schuchat said.