Part A was primarily designed to determine the pharmacokinetics (PK) of suptavumab in infants to inform the dose regimen used in Part B of the study. The study protocol specified the process and criteria for assessment of the dose. The dose used in Part B was to remain the same as Part A if the PK data up to Day 57 demonstrated that the individual PK observations were consistent with model-predicted concentrations, following age and body weight corrections.

A medically attended RSV infection defined as an infant with positive RSV test by Reverse-transcriptase polymerase chain reaction (RT-PCR) with any of following events: Hospitalized (on basis of assessment of admitting physician) for RSV infection or outpatient visit (emergency room \[ER\], urgent care \[UC\], or pediatric clinic visits \[for either a sick or well visit\]) with RSV lower respiratory tract infection (LRTI). An RSV LRTI in an infant: RSV proven respiratory infection (i.e positive RSV RT-PCR test) with parent(s)/guardian(s) report of cough/difficulty breathing, \& with 1 of following signs of LRTI, as assessed by healthcare provider: - lower chest wall in drawing -hypoxemia (peripheral capillary oxygen saturation \<95% breathing room air) - Wheezing/crackles. The 150-day efficacy assessment period: first study drug intake through the Day 150 visit.