Surveillance for poliovirus vaccine adverse events, 1991 to 1998: Impact of a sequential vaccination schedule of inactivated poliovirus vaccine followed by oral poliovirus vaccine

Background. The elimination of wild-virus-associated poliomyelitis in the W estern Hemisphere in 1991 and rapid progress in global polio eradication ef forts changed the risk-benefit ratio associated with the exclusive use of o ral poliovirus vaccine (OPV) for routine immunization. These changes, plus the November 1987 development of an enhanced-potency inactivated poliovirus vaccine (IPV), which poses no risk of vaccine-associated paralytic poliomy elitis (VAPP), resulted in a change in polio immunization policy in the Uni ted States. In September 1996, the Centers for Disease Control and Preventi on recommended that IPV replace OPV for the first 2 doses in a sequential p oliovirus vaccine schedule. The Vaccine Adverse Event Reporting System (VAE RS), a passive surveillance system for adverse events after receipt of any US-licensed vaccine, is used to monitor postlicensure vaccine safety. Postl icensure surveillance of vaccines is important to identify new, rare, or de layed-onset adverse reactions not detected in prelicensure clinical trials or when new vaccine schedules are adopted. Through continual monitoring of adverse events and identification of potential vaccine risks, VAERS can ser ve as an important resource to ensure continued public acceptance of vaccin es. We compared VAERS reports after the receipt of IPV to reports after OPV in infants from 1991 through 1998. Comparisons included reports listing IP V and OPV coadministered with other vaccines. Methods. Annual reporting rates per 100 000 doses distributed within 3 seve rity categories (fatal, nonfatal serious, less serious) were examined. Dist ributions of severity categories by vaccine type, age, and time period (pre - and postrecommendation) were constructed. Safety profiles (distribution o f 21 symptom groupings) for IPV and OPV reports were compared. Analysis was restricted to reports for infants 1 to 3 months old and 4 to 6 months old, corresponding generally to first- and second-dose recipients. Any notable increase in a severity or safety category for IPV compared with OPV was fol lowed up by examining the frequency of specific symptoms, reporting source, and date of vaccination. An important limitation of VAERS is that reports do not necessarily represent adverse events caused by vaccines. In many cas es, the events are temporal associations only. Results. The annual rates of VAERS reports per 100 000 vaccine doses distri buted by severity category, 1991 to 1998, were in general similar for repor ts after IPV compared with those after OPV. The reporting rates for poliovi rus vaccine did not increase materially with the shift to IPV usage. The re lative frequencies of symptoms in the fatal and nonfatal serious categories for 1998 vaccine administrations were similar to 1997 reports. Severity pr ofiles for IPV and OPV reports in infants 1 to 3 months old and 4 to 6 mont hs old, corresponding to first- and second- dose recipients, were remarkabl y similar. The frequency of symptoms listed on IPV reports categorized as f atal or serious was examined by age, vaccine combinations, and time period, and the distribution of symptoms was similar for ages 1 to 3 months and 4 to 6 months. In the postrecommendation period, the 10 most frequent symptom s reported with IPV were also reported with OPV in either similar or lower relative frequency. During the postrecommendation period, safety profiles f or infants 4 to 6 months old showed a 2.5% higher proportion in the allergi c reaction category for IPV than for OPV, but none of the allergic reaction reports indicated anaphylaxis. In general, the distribution of symptom gro upings was not markedly different for IPV compared with OPV. No cases of VA PP were reported after the administration of IPV, whereas 5 VAPP cases were reported after the administration of OPV. Conclusions. Although VAERS is subject to the limitations of most passive s urveillance systems, the large number of reports and national coverage prov ide a unique database for monitoring vaccine safety. There was a marked inc rease of IPV reports in VAERS after 1996, consistent with implementation of the Advisory Committee on Immunization Practices recommendation for the se quential IPV/OPV poliovirus vaccination schedule. Given the increased use o f IPV, a review of potential adverse events in VAERS compared IPV with OPV reports both before and after the introduction of the sequential vaccinatio n schedule. Vaccine safety surveillance indicated no adverse events pattern s of potential concern following the use of IPV in infants after the introd uction of the sequential vaccination schedule. Ongoing surveillance is docu menting a decrease in VAPP. These findings provide useful information to su pport the Advisory Committee on Immunization Practices recommendation, made in 1999, to shift to an all-IPV schedule.