The Centers for Disease Control and Prevention (CDC) began passively measuring immunization coverage in 1962 using vaccine manufacturers reports and the Biologics Surveillance System. The vaccine manufacturers reports measures the number of vaccine doses distributed but does not take into account unused or wasted vaccine, overimmunization, or the age at which the vaccine was administered. Biologics Surveillance System monitors government vaccine purchases, which is currently about 55% of childhood vaccines. Neither of these systems can generate accurate population coverage estimates.