Immune Thrombocytopenia as an Emerging Adverse Event with Nirsevimab : A pharmacovigilance signal detection study using the Food and Drug Administration Adverse Event Reporting System database (FAERS)

Introduction Respiratory syncytial virus (RSV) is a major cause of acute lower respiratory tract infections (LRTIs) in infants and young children worldwide and contributes to childhood morbidity, hospitalisation, and mortality. Nirsevimab is a long-acting monoclonal antibody approved for prevention of RSV infection. Clinical trials have reported rash and injection site reaction as common adverse events encountered. Continued post-marketing pharmacovigilance is important to assess the potential risk of immune thrombocytopenia, given its biological plausibility. This study evaluated disproportionate reporting of ITP associated with nirsevimab using FAERS database. Methodology A disproportionality analysis was performed using FAERS quarterly data from Q3 2023 to Q2 2026. Reports in which nirsevimab was the primary suspect drug and using MedDRA Preferred Term for adverse event “Immune thrombocytopenia” were identified. The disproportionality between the drug – adverse event pair was assessed using Reporting Odds Ratio (ROR), Proportional Reporting Ratio (PRR), chi-square statistic, Information Component (IC), Bayesian confidence propagation neural network (BCPNN) and multi-item gamma Poisson shrinker (MGPS) methods were assessed. A signal was confirmed when ROR >2 with a lower bound of 95% Confidence interval (CI)>1, PRR >2, chi-square >4, IC-2SD>0, EBGM05>2. Results Among FAERS reports, nirsevimab was the primary suspect drug in 2,244 reports, while ITP was reported in 2,585 reports involving any suspect drug. Fourteen reports listed nirsevimab as the primary suspect drug and immune thrombocytopenia as the recorded reaction, concentrated in four of twelve quarters. All four algorithms detected a signal: ROR 11.89 (95% CI 7.02–20.14), PRR 11.82 (lower 95% CI 7.00–19.96; χ 2 = 138.04, p < 0.0001; IC 3.56), BCPNN: IC − 2SD = 2.9052, and MGPS: EBGM05 = 6.9467. Conclusion The magnitude of the disproportionality is substantial enough to warrant designation as a potential signal and is concordant with a recent FDA identified potential signal. Detailed medical record review, active surveillance and international pharmacovigilance collaboration is required for establishing causal relationship.