Importance of background information of the national immunization program for children in vaccine signal detection

Background/Introduction: Annually Pharmacovigilance Center Lareb receives a large number reports of Adverse Events Following Immunisation (AEFIs) after childhood vaccination. Usually, these concern reports of well-known AEFIs. With the current methods for signal detection, being case-by-case analyses and statistical screening of the database, potential signals related to the vaccination scheme of the National Immunization Program (NIP) are less likely to be detected. Detailed information of the NIP, like the number of vaccinees, administered vaccines, batch numbers, per vaccination moment is known. Amendments of the NIP such as changes in the administered vaccine over time are well documented. Current signal detection strategies, may profit from applying this background information. Objective/Aim: To illustrate the possibility to detect new signals in wellknown AEFIs by using by using background information of the NIP. Methods: Based on a multiannual overview 2011-2017 of reported AEFIs after administration of DTPa-IPV (Diphtheria, acellulair Pertussis, Tetanus, inactivated Poliovirus) booster vaccine at the age of 4, we investigated the added value of using background information (number of vaccinees and changes in administered vaccines) in signal detection. Results: Reports of AEFIs after administration of DTPa-IPV at the age of 4 years, in the period 2011-2017 were selected and by using the exact number of vaccinees the reporting rate per 10.000 vaccinees were calculated. A graphical overview showed an increase in 2015 and a sharp decrease in 2017. The decrease was outspoken in reports of extensive swelling of the vaccinated limb (ELS). The decrease in 2017 appeared to be linked to the replacement of Infanrix- IPV® by Boostrix Polio® in in the NIP in the spring of 2017. ELS was more frequently reported after administration of the brand Infanrix-IPV® compared to Boostrix Polio® (OR 9.1, 95% CI 5.1-16.2). The increase in 2015 may be related to a vaccine change in the primary vaccine series at infant age. Children primed with Infanrix hexa® reported more frequently an AEFI after administration of Infanrix-IPV® at the age of 4 compared to children primed with Pediacel® (OR 2.0; 95% CI 1.9-2.2). Conclusion: By using background information of the program, two signals were detected related to well-known AEFIs. Using background information of the NIP is of added value in vaccine signal detection.