Objective.-To evaluate whether immunization against a vaccine-preventable disease is sought to avoid the naturally occurring disease itself, we hypothesized that the rate of ''on-time'' measles immunization would increase during an epidemic of that disease. If such an effect occurred, we wondered whether it would have an impact on on-time administration of other recommended immunizations. Design.-Retrospective evaluation of immunization rates of children at their second birthday with the use of computerized health records for children entering kindergarten in an 8-year interval spanning the onset of epidemic measles in Chicago, III, in 1989 and 1990. Setting.-Children entering Chicago public schools. Main Outcome Measures.-Rates of receipt of measles-containing vaccine (MCV), 1 to 4 doses of a diphtheria toxoid-tetanus toxoid-pertussis (DTP) or diphtheria toxoid-tetanus toroid (DT) vaccine, 1 to 3 doses of oral or inactivated polio vaccine (OPV/IPV), and the full series of these vaccines (4:3:1) that are required to be ''up-to-date'' by the second birthday. Results.-The rate of on-time MCV receipt increased from 56% to 58% in the years prior to the epidemic to 70% during the epidemic (1989 and 1990). A similar increase did not occur for DTP/DT 4 or OPV/IPV 3. Moreover, among older children delayed in MCV receipt, evidence of catch-up immunization also occurred during the epidemic years; similar catch-up for delayed DTP/DT 4 or OPV/IPV 3 immunization did not occur. Conclusions.-Dramatic increases in on-time and catch-up MCV receipt occurred during the Chicago measles epidemic of 1989 and 1990. The lack of similar increases in DTP/DT 4 and OPV/IPV 3 suggests MCV receipt was not associated with receipt of other recommended immunizations during that time.
