In 2002, just 63 percent of toddlers in Colorado were getting the protective shots on time. That was the worst performance in the nation. Massachusetts was doing the best job, immunizing 86 percent of toddlers on time.
Because vaccination coverage varies so widely by state, that means tens of thousands of children every year are left unprotected, said Dr. Walter Orenstein of the Centers for Disease Control and Prevention.
"If we let down our guard ... vaccine preventable diseases will return," he warned. He said it's not clear why some states do much better than others.
The CDC, Oreinstein said, is encouraging states to try to identify specific areas where children are unprotected and to try programs such as computer-based registries that sends parents and doctors reminders when a child misses a scheduled shot.
Government health officials recommend that by age 2 children receive about 20 doses of vaccine against 11 diseases.
Nationwide, the CDC reported good news: Vaccinations against chickenpox are steadily rising, to a high of 81 percent coverage in 2002.
For the nation's newest vaccination, the pneumococcal vaccine that protects against meningitis and ear infections, 41 percent of children had up-to-date shots. This is the first time the CDC has counted success with this vaccine.