SAN FRANCISCO (KGO) -- The flu has officially surpassed the epidemic threshold this season. The flu is now widespread in 36 states, but so far, California has avoided significant activity.
Part of the problem this year is the flu strain mutated in ways to make the flu vaccine less effective than experts hoped.
Flu activity usually peaks between December and February, but it can last as late as May.
At a scientific meeting at the CDC in October, vaccine experts were told of preliminary results from three studies that found AstraZeneca's FluMist nasal spray had little or no effect in children against the swine flu strain that was the most common bug making people sick last winter.
Because this year's version of FluMist is the same formulation, experts said it's possible the spray vaccine won't work for swine flu this season, either.
However, CDC officials believe H3N2 will be the most common flu bug this winter.
Common flu now at epidemic levels in the US