Objective: To inform policies about possible immunization, this literature-based model sought to estimate the adverse effects burden of administering nirsevimab, a passive protection against RSV in infants.

Methods: If licensed, nirsevimab could be administered during the birth hospitalization for infants born during the RSV season (October-February). Infants born outside that season would require a separate outpatient visit. The product related adverse effects were calculated by pooling nirsevimab safety data from phases 2 and 3 trials to obtain rates of each non-target adverse event (AE). Each infant and family faced non-product related AEs: a 3.17% increased risk of influenza and 6.82% added COVID-19 risks from each additional well-child visit needed by some infants born outside the RSV season. Data came from Simmering et al, 2014, CDC surveillance data, and the Global Burden of Disease (GBD) Study (burden per consequence).

Results: Nirsevimab immunization resulted in adverse effects disability rate of 6.8 DALYs per 100,000 in-season birth infants and 178.2 DALYs per 100,000 out-of-season birth infants. This shows a 26-fold higher burden to out-of-season infants. These burdens are comparable to the DALY burden per 100,000 population for infant thalassemias trait (5) and infant meningitis (201), respectively. The out-of-season birth infant burden rate was mainly driven by COVID-19 and influenza cases. This is the result of additional well-child visits for infant immunization with appreciable estimated risks--6,816 COVID-19 and 3,170 influenza cases per 100,000 out-of-season infants immunized. The distribution of factors contributing to AE the DALY burden per 100,000 infants were COVID-19 (120, 67%), influenza (51.7, 29%) and nirsevimab (6.8, 4%).

Conclusion: Due to possible infant and family exposure to influenza and COVID-19 from added health care visits, adverse effects of nirsevimab immunization to out-of-season low-risk births are substantial. Policy makers will need to weigh risks and benefits carefully for this group of infants.