Predictors of Serious Adverse Events Following Immunization: 1-Year Pharmacovigilance Study in General Population

Background/Introduction: An adverse event following immunization (AEFI) is a clinical occurrence which follows vaccination and does not necessarily have a causal relationship with vaccine use. Public health authorities of high-income countries noted an increase in number of vaccine hesitant patients/parents, based on presumed severe side effects. Objective/Aim: The aims of this study were to describe frequency, pre- ventability and seriousness of AEFI in relation to the number of administered doses, and to identify predictors of AEFI seriousness. Methods: This is a retrospective study performed on reports of suspected AEFI collected in Tuscany (Italy) between January and December 2017. Patients’ demographics and clinical status, suspected vaccines, AEFIs description were collected. Preventability and causality were assessed using the Schumock and Thornton and the World Health Organization algorithms, respectively. Logistic regression was used to estimate the reporting odds ratios of potential predictors of AEFI seriousness. Results: We observed 570 AEFIs on a total of 695769 doses administered (0.82/1000 doses). 139 AEFIs were reported as serious, of them 83 led to hospitalization. Reports were mostly related to one vaccine, and a total of 2–5 antigens (n = 364). Among reports with more than one vaccine, only 14 AEFIs presented a potential drug–drug interaction. The most frequently reported AEFI was ‘‘general disorders and administration site conditions’’. The most frequently reported vaccine was Meningococcus B. Risk of serious AEFI was significantly higher in males (p = 0.036). The total number of administered antigens did not correlate with AEFI seriousness (p = 0.940). Overall, only 46 AEFIs (8.1%) were preventable, and the majority of them (n = 375) were consistently related with the immu- nization. Considering the number of administered doses, the highest rate of AEFI was observed for Pneumococcus vaccine (5.70/1000 doses). Conclusion: The rate of AEFIs in Tuscany was considerably low and the majority of them was not serious (75.6%) and not preventable (91.9%). The number of administered antigens did not influence the risk of reporting a serious AEFI, confirming the safety of combination vaccines. Pharmacovigilance represents the best strategy to estimate and charac- terize the burden of AEFIs in general population, and supports healthcare professionals in addressing patients/parents’ concerns on vaccines rec- ommended in Italian immunization programs.