BACKGROUND:
1 in 4 US children do not receive all recommended vaccinations by age 2 years. One cause of under-immunization is the deferral of doses when too many are due at a single visit.

OBJECTIVE:
To examine the frequency of vaccine dose deferrals and the relationship between deferral and immunization coverage rates.

METHOD:
The study was conducted in 32 office-based pediatrics centers, all of which routinely administered pneumococcal conjugate vaccine. Records were retrospectively reviewed for patients whose first outpatient visit was to the study center at age <2 months and who continued to be seen by the center to age 18+ months. For each vaccination visit during ages 2-8 months, we determined the number of vaccines doses due (based on ACIP minimum age and dose interval recommendations) and the number administered. The occurrence of 1+ deferred dose and immunization coverage at age 2 years was considered using logistic regression. Adjusted odds ratios (ORs) were calculated using a model considering patient gender, race/ethnicity, health insurance, and the number of vaccination visits during ages 2-8 months.

RESULT:
1+ doses were deferred at 37% of 2,224 vaccination visits during ages 2-8 months. The rate of deferral was 26%, 34% and 48% when 3, 4 and 5 vaccines were due, respectively (p<001). Having no deferred dose was strongly associated with improved coverage. Among children with no deferred dose, 80% received 3+ doses of each of DTaP, IPV, HIB, PCV and HBV by age 2 years, vs 73% of children with 1+ deferred doses (OR 2.0, 95% CI 1.4-2.9). Other factors significantly associated with improved coverage were non-Hispanic white race/ethnicity (OR 1.8, 95% CI 1.3-2.6), private health insurance (OR 1.9, 95% CI 1.3-2.8), and having at least 3 immunization visits during ages 2-8 months (OR 2.8, 95% CI 2.0-4.0).

CONCLUSION:
In the next decade, both new vaccines and new combinations of existing vaccines will likely become available. Our data indicate combination vaccines may play a significant role in increasing coverage rates.

LEARNING OBJECTIVES:
To understand the relationship between the number of vaccine injections due and the risk of deferring doses, and the effect of such deferrals on coverage rates.