Volume matters: physician practice characteristics and immunization coverage among young children insured through a universal health plan.

OBJECTIVES: We studied the association between immunization coverage for a cohort of 2-year-old children covered by a universal health insurance plan and pediatric provider and other health services characteristics.
METHODS: We assembled a cohort of 101,570 infants born in urban areas in Ontario, Canada, between July 1, 1997, and June 31, 1998. Children were considered to have up-to-date (UTD) immunization coverage if they had > or =5 immunizations by 2 years of age, ie, the recommended 3 doses and 1 booster of diphtheria-polio-tetanus-pertussis/Haemophilus influenzae type b vaccine and 1 dose of measles-mumps-rubella vaccine. Provider practice characteristics were derived from outpatient billing records, and 1996 census data were used to derive neighborhood income quintiles. The association between UTD immunization status and provider characteristics was assessed with multilevel regression models, controlling for patient characteristics.
RESULTS: Overall, the rate of complete UTD immunization coverage was low (66.3%) despite a large number of primary care visits (median: 19 visits). Children whose usual provider had a low volume of pediatric primary care were less than one half as likely to be UTD. Other factors associated with not being UTD included very low continuity of care, low continuity of care, and usual provider in practice for <5 years. With adjustment for patient and provider characteristics, there was no difference in immunization coverage for general practitioners versus pediatricians. Children from low-income neighborhoods were less likely to be UTD.
CONCLUSIONS: Despite universal access to primary care services, rates of complete immunization coverage among 2-year-old children in Ontario are low. Because visit rates are high, primary care reform should include interventions directed at provider immunization practices to reduce missed opportunities.