The Centers for Disease Control and Prevention (CDC) said last week that the national immunization rate among children is getting better, but Colorado has been named the worst in the United States in terms of vaccination rates for diseases like polio, measles, and hepatitis B. The state has a vaccination rate for children aged three years or younger of just 62 percent, compared with the national leader Massachusetts, which has a rate of 86 percent, and the national average of 75 percent. Observers say a number of factors have dropped Colorado's immunization rate, including a lack of state matching for federal vaccination campaign funding, low reimbursement rates from Medicaid in Colorado, the ease with which parents in Colorado can receive waivers from their schools to avoid vaccine requirements there, and the lack of a statewide tracking system to monitor which children have or have not received vaccines.