Nirsevimab serum concentrations versus time after dose.
The blue and red lines/dots represent the nirsevimab serum concentrations from two participants with resistance-associated substitutions N208S and I64T:K68E who both weighed ≥5 kg and received the 50 mg dose in the Phase 2b trial. The gray lines/dots denote individual serum concentration-time profiles from participants in the Phase 2b trial <5 kg and MELODY (primary cohort) who had received weight-banded dosing; the broken lines are the 5th and 95th percentiles of the data. Samples below the limit of quantification (0.5 µg/mL) are shown as 0.5 µg/mL.Susceptibility to nirsevimab increased for RSV B isolates with the common I206M:Q209R substitution, as well as for additional substitutions co-occurring with I206M:Q209R (Tables 