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Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infection in young children. Motavizumab is an investigational humanized monoclonal antibody for RSV prophylaxis.A dose-escalation study was conducted followed by assessment of safety, tolerability, serum concentrations, and immunogenicity during a second consecutive RSV season. In season 1, premature infants aged ≤6 months or children ≤24 months with chronic lung disease of prematurity received monthly motavizumab (3 or 15 mg/kg). In season 2, children who received ≥3 motavizumab doses in season 1 were randomized to receive monthly motavizumab or palivizumab 15 mg/kg.Of 217 children enrolled in season 1, 211 (97.2%) received motavizumab 15 mg/kg and 205 (94.5%) patients completed the study through 90 days after the final dose. In season 2, 136 children were randomized to receive motavizumab (n = 66) or palivizumab (n = 70). The most commonly reported related adverse event was transient injection site erythema. In season 1, mean trough motavizumab concentrations were 7.9 and 50.2 μg/mL after the 3- and 15-mg/kg doses, respectively. Trough concentrations increased with repeated motavizumab dosing; a similar pattern was seen in season 2. Antimotavizumab reactivity occurred infrequently (3.3%) in season 1. In season 2, no treatment group–specific antidrug antibody was detected through 90 to 120 days after dosing with either product.The pharmacokinetic profile of motavizumab was similar to that of other IgG1 antibodies. Increased adverse reactions or immunogenicity were not observed during and after a second season of treatment with motavizumab. Safety findings from these studies supported the continued development of motavizumab.