Impact of the change in polio vaccination schedule on immunization coverage rates: A study in two large health maintenance organizations

Objective. In January 1997, one of the most significant changes to United S tates vaccine policy occurred when polio immunization guidelines changed to recommend a schedule containing inactivated polio vaccine (IPV). There wer e concerns that parent or physician reluctance to accept IPV into the routi ne childhood immunization schedule would lead to lowered coverage. We deter mined whether adoption of an IPV schedule had a negative impact on immuniza tion coverage. Design. A cohort study of 2 large health maintenance organizations (HMOs), Group Health Cooperative and Kaiser Permanente Northern California, was con ducted. For analysis at 12 months of age, children who were born between Oc tober 1, 1996, and December 31, 1997, and were commercially insured and cov ered by Medicaid were continuously enrolled; for analysis at 24 months of a ge, children who were born between October 1, 1996, and June 30, 1997, and were commercially insured and covered by Medicaid were continuously enrolle d. The 3 measures of immunization status at 12 and 24 months of age were up -to-date status, cumulative time spent up-to-date, and the number of missed opportunity visits. Results. At both HMOs, children who received IPV were as likely to be up to date at 12 months as were children who received oral poliovirus vaccine (O PV), whereas at Group Health, children who received IPV were slightly more likely to be up to date at 24 months (relative risk: 1.12; 95% confidence i nterval [CI]: 1.05, 1.19). These findings were consistent for children who were covered by Medicaid. At Kaiser Permanente, children who received IPV s pent;3 fewer days up to date in the first year of life, but this difference did not persist at 2 years of age. At Group Health, children who received IPV were no different from those who received OPV in terms of days spent up to date by 1 or 2 years of age. At Group Health, children who received IPV were less likely to have a missed opportunity by 12 months old (odds ratio [OR] 0.46; 95% CI: 0.31, 0.70), but this finding did not persist at 24 mon ths of age. At Kaiser Permanente, children who received IPV were more likel y to have a missed opportunity by 12 months (OR 2.06; 95% CI: 1.84, 2.30), and 24 months of age (OR 1.50; 95% CI: 1.36, 1.67). Conclusions. The changeover from an all-OPV schedule to one containing IPV had little if any negative impact on vaccine coverage. Use of IPV was assoc iated with a small increase in the likelihood of being up to date at 2 year s of age at one of the HMOs and conversely was associated with a small incr ease in the likelihood of having a missed-opportunity visit in the other HM O.