Evaluation of the safety of palivizumab in the second season of exposure in young children at risk for severe respiratory syncytial virus infection

Background: Palivizumab reduces respiratory syncytial virus (RSV) hospitalisations in high-risk infants. Those with severe bronchopulmonary dysplasia may require two seasons of prophylaxis. There is concern that this humanised antibody might cause an adverse immune response in a second season of use. Objective: To evaluate and compare the occurrence of anti-palivizumab antibodies and clinical adverse events in subjects receiving monthly palivizumab injections for a first and second season, and to assess frequency and severity of RSV disease in the two groups. Design and Patients: Subjects aged less than or equal to2 years at severe risk for RSV disease were designated as first season (no previous palivizumab exposure) or second season subjects (received palivizumab in previous RSV season). Palivizumab injections (15 mg/kg) were administered monthly for up to 5 months. Anti-palivizumab antibody titres and serum palivizumab concentrations were measured; adverse events were recorded. Results: No first (n = 71) or second (n = 63) season subjects experienced a significant anti-palivizumab antibody response (titre greater than or equal to1 : 80). Serum palivizumab concentrations were similar for the two groups. Nine (12.7%) first season and 8 (12.7%) second season subjects experienced one or more serious adverse events; most were respiratory and all were considered to be not or probably not related to palivizumab. No deaths occurred during the study. Conclusions: Monthly palivizumab injections were not associated with adverse immune responses or adverse events in young children receiving palivizumab for one or two seasons. Children receiving palivizumab for a second season did not experience more severe adverse events than those receiving it for the first time.