Nevada County has the lowest childhood immunization rates among California’s 58 counties, but the system that determines compliance with school vaccine mandates is somewhat misleading, said parents and nurses. “If I want to say that I don’t want to give my kids four shots at once, if you break from that schedule, then you have to sign the back of that (opt-out) card,” said Melissa Sampley, a Penn Valley mother. “That goes into the database as exempt from vaccination.” When students enter kindergarten, their parents must demonstrate they have had the required shots, including vaccines for polio, pertussis, measles, mumps, …