Objective. To investigate trends over time in polio extraimmunization among children in 4 large health maintenance organizations and to study the association with recent changes in polio immunization policy.

Methods. Using 176 169 children who were born after 1994 and enrolled for their first 2 years of life, we assessed rates and trends of polio extraimmunization in the Vaccine Safety Datalink project. We used logistic regression to test the association of extraimmunization with different polio immunization schedules and with sociodemographic characteristics and used Poisson regression to test changes in rates over time.

Results. Overall, 10.5% were extraimmunized for poliovirus; children on the all inactivated polio virus or sequential schedule were one half as likely as those on the all oral polio virus schedule to be extraimmunized by 2 years of age. There was a significant decrease in extraimmunization over time, with <5% of children born at the end of 1997 being extraimmunized, compared with >15% at the beginning of 1994.

Conclusions. Poliovirus extraimmunization rates have fallen dramatically in association with the change-over to the all inactivated polio virus schedule.

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