The Effectiveness of Early Immunisation With Nirsevimab on Preschool Wheezing in France, Based on an Analysis of Data From the French Health System Database.

Early respiratory syncytial virus (RSV) bronchiolitis is a well-known contributing factor of mid- and long-term respiratory sequelae in children such as recurrent lower tract respiratory infections (LRTIs), preschool wheezing (PW) or decreased lung function at older ages. Nirsevimab, a monoclonal antibody against RSV with enhanced neutralizing activity and a prolonged half-life, has shown great potential in reducing short term outcomes such as hospitalization for RSV associated bronchiolitis (estimated adjusted effectiveness against hospitalization to 83.0% (95%CI: 73.4 to 89.2). Other countries that have been using nirsevimab almost exhaustively have reported similar results such as in Galicia. However, prevention of RSV LRTI or delay RSV infection early in life has not been investigated on respiratory manifestations such early wheezing in infancy (ie before 2 years of age) and recurrent wheezing in preschool period (ie between 2 and 6 years old). Some recent data have shown that nirsevimab was associated with a reduction of wheezing in the year following its administration (HR: 0.73; 95% CI, 0.58-0.93). Other data have shown that not being infected by RSV during the infancy was associated with 26% lower risk of 5-year current asthma than being infected (5) giving good rational for preventive strategies against RSV to impact midterm respiratory outcomes such as PW and then asthma. Ongoing observational cohort studies will help investigate the question of mid-term effects, but they will come with a high costs and risk of attrition. Administrative national health claims database could prove to be useful and complementary by studying these types of real world outcomes across lives of children that have been immunized by nirsevimab as shown recently. Therefore, the investigators aim to study the impact of early nirsevimab administration on wheezing manifestations occurring in infancy and beyond during the preschool years. Our primary objective is to see if nirsev