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.

The PK analysis set included participants who received a single dose of suptavumab and had at least 1 measurable concentration of suptavumab in serum. Here "Number Analyzed" signifies those participants who were evaluable for this outcome measure at specific time point.

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.

Full analysis set (FAS) included all randomized participants who received any study drug and was analyzed according to treatment allocated by Interactive voice response system (IVRS)/ Interactive web response system (IWRS) at randomization (as randomized).

Any untoward medical occurrence in participants, who received investigational medicinal product (IMP) was considered an adverse event (AE) without regard to possibility of causal relationship with this treatment. TEAEs: AEs that developed/worsened/became serious during on-treatment period (defined as time between the date of first study drug administration & date of end of study/last visit).Serious AE: Any untoward medical occurrence that resulted in any of following outcomes: death, life-threatening, required initial or prolonged in-patient hospitalization, persistent/significant disability/incapacity, congenital anomaly/birth defect, or considered as medically important event. Any TEAE included participants with both serious & non-serious AEs. National Cancer Institute Common Terminology Criteria (NCI-CTCAE) version 4.03(Grade 3 [severe] & Grade 4[life-threatening]) was used in this study to grade clinical AEs.

Safety analysis set (SAF) included participants who received any dose of suptavumab.

Serum samples for drug concentration will be collected at pre-specified time points

The PK analysis set included participants who received a single dose of suptavumab and had at least 1 measurable concentration of suptavumab in serum. Here "Number Analyzed" signifies those participants who were evaluable for this outcome measure at specific time point.

ADA category of each participant was classified as pre-existing immunoreactivity (a positive ADA response at baseline with a <4-fold increase in titer for all post baseline samples), treatment-boosted (a positive response at baseline with at least one post baseline titer at >=4-fold the baseline titer), or treatment-emergent (TE [any positive post baseline assay response when baseline results were negative or missing]). TE ADA responses were further classified as persistent (treatment-emergent positive ADA response detected in at least 2 consecutive post baseline samples separated by at least a 12-week post baseline period [based on nominal sampling time], with no ADA-negative samples in-between, regardless of any missing samples or a positive response at the last ADA sampling time point), indeterminate (a positive assay response at the last collection time point only, regardless of any missing samples), or transient (not persistent/indeterminate, regardless of any missing samples).

The ADA analysis set contained participants who received a single dose of suptavumab and had at least 1 post-treatment ADA result.

A medically attended RSV infection was defined as an infant with a positive RSV test by RT-PCR with any of the following events: -Hospitalized (on the basis of the assessment of the admitting physician) for RSV infection - or Outpatient visit (ER, UC), or pediatric clinic visits [for either a sick or well visit]) with RSV 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 or difficulty breathing, and with 1 of the following signs of LRTI, as assessed by a healthcare provider: -Lower chest wall indrawing -Hypoxemia (peripheral capillary oxygen saturation <95% breathing room air) -Wheezing or crackles. The 150-day efficacy assessment period:first study drug intake through the Day 150 visit.

Analysis was performed on FAS population.