Picking the virus strains that a flu shot will cover is like a scientific guessing game.

Backer: “Those have to be chosen, a year, a whole year in advance of the flu season given the lead time required for vaccine production.” (:10)
 
That’s Dr. Howard Backer with California’s Department of Public Health. He says flu cases are up around the state and one reason is because this year’s vaccine does not match all the viruses being reported.

Dr. Glennah Trochet, of Sacramento County’s Health Department, says a shot can’t cover every flu virus.

Trochet: “I’m always surprised when it is an exact match because these viruses are constantly mutating and changing.” (:06)
 
The Centers for Disease Control and Prevention estimates that this year’s vaccine only covered half of the flu cases reported nationally.