Hypotonia and Lethargy as Potential Adverse Effect to Nirsevimab Administration in a Newborn Infant.

Respiratory syncytial virus (RSV) is one of the most common causes of acute lower respiratory tract infections in children. Immunization with nirsevimab has had a positive impact in reducing the number of emergency department (ED) visits for RSV. Clinical trials have shown that most adverse events associated with nirsevimab administration were mild, with no significant difference compared with placebo. We report on a 13-day-old female newborn who was brought into the ED because of hypotonia and unresponsiveness after administration of nirsevimab. During the 48-hour hospital stay, the only relevant clinical finding was mild axial hypotonia, which gradually resolved without treatment. A cerebral function monitor showed a continuous normal voltage tracing with no signs of seizures, and a complete electroencephalogram revealed normal background activity. Given that all the investigations were normal and the main event preceding the events was the administration of immunoprophylaxis with nirsevimab, a possible association between the clinical symptoms and the medication was suggested. Although there is robust evidence to recommend nirsevimab for reducing hospitalizations, we must bear in mind that, as a relatively new drug, we believe it is essential to emphasize continued monitoring of side effects, particularly in neonates.