The Sutter Pacific medical Foundation says some vaccines were not stored at the proper temperature, so they may not be effective.
Officials are asking anyone who got vaccinated at one of its care centers in San Francisco, Sonoma, Marin or Lake counties to return for another shot.
They say the original vaccines don't have any harmful effects, but the patients may not be immunized.
The Sutter Pacific number to schedule an appointment or speak with a nurse regarding the revaccination is 1-877-657-8987.
