The successful completion of early childhood immunizations is a proxy for overall quality of early care. Immunization statuses are usually assessed by up-to-date (UTD) rates covering combined series of different immunizations. However, series UTD rates often only bear on which single immunization is missing, rather than the success of all immunizations. In the US, most series UTD rates are limited by missing fourth DTaP-containing immunizations (diphtheria/tetanus/pertussis) due at 15 to 18 months of age. Missing 4th DTaP immunizations are associated either with a lack of visits at 15 to 18 months of age, or to visits without immunizations. Typical immunization data however cannot distinguish between these two reasons. This study compared immunization records from the Oregon ALERT IIS with medical encounter records for two-year olds in the Oregon Health Plan. Among those with 3 valid DTaPs by 9 months of age, 31.6% failed to receive a timely 4th DTaP; of those without a 4th DTaP, 42.1% did not have any provider visits from 15 through 18 months of age, while 57.9% had at least one provider visit. Those with a 4th DTaP averaged 2.45 encounters, while those with encounters but without 4th DTaPs averaged 2.23 encounters. 1. Introduction Timely immunization receipt is commonly assessed by up-to-date (UTD) rates for a combination of vaccine series, such as the 4？:？3？:？1？:？3？:？3？:？1 series recommended between birth and 18 months of age by the American Academy of Pediatrics (AAP) and the Advisory Committee for Immunization Practice (ACIP). The 4？:？3？:？1？:？3？:？3？:？1 series contains 4 DTaP (diphtheria/tetanus/pertussis), 3 IPV (polio), 1 MMR (measles/mumps/rubella), 3 HBV (hepatitis B), 3 Hib (Haemophilus influenzae), and 1 varicella vaccines. The need for four early DTaP immunizations is due to the continual reoccurrence of pertussis in the US, its risk to infants, and the difficulty in building adequate disease protection to pertussis [1]. Children who are not complete for their early childhood shots generally lack only one visit or shot, and the commonly missing component is the fourth DTaP due between 15 and 18 months of age [2, 3]. The lack of a fourth DTaP frequently holds down series UTD rates even when all other vaccines are well delivered. The UTD rate for completing four DTaPs as measured by the National Immunization Survey by 19 to 35 months of age lags behind most other early childhood immunizations [4]. Immunization UTD rates are used as measurable proxies for the overall receipt of appropriate early medical care and well-child visits [5]. The DTaP
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