Do too many shots due lead to missed vaccination opportunities? Does it matter?

Background Missed childhood vaccination opportunities have been generally described, yet not since the immunization scheduleʹs recent rapid expansion. Little is known about the relationship between the number of vaccine doses due and whether all scheduled doses are administered, and the effect of dose deferral on immunization coverage. Methods 32 private pediatrics centers reviewed medical records covering the first 2 years of life for 858 patients. For each visit during ages 2–8 months, we determined the numbers of vaccine doses due versus administered. Logistic regression was used to assess the effect of dose deferral on immunization coverage at ages 1 and 2 years. Results Of 2224 visits during ages 2–8 months at which ≥1 dose was administered, ≥1 due dose was deferred at 26%, 34%, and 48% of the visits at which ≤3, 4, and 5 doses were due, respectively. Absence of a deferred dose visit predicted increased coverage at age 1 (adjusted odds ratio: 2.4, 95% confidence interval: 1.8–3.2) and 2 years (2.1, 1.4–3.0). Conclusions Administering some but not all vaccine doses at visits during ages 2–8 months impairs immunization coverage through age 2 years.