Following changes are made: • The status of the FIH study was updated by removing “ongoing” • Clarified the primary objective for Part B • Clarified collection of days on supplemental oxygen • Clarified, in Part B, that the permitted windows for prophylaxis will be based on geographical area • Removed references to the study manual • Clarified, in Part A, the process of dosing for the 24 subjects • Clarified physical examination assessment time points, including the addition, in Part A and Part B, of a physical examination postdose • Clarified, in Part A, whether Visit 4 through Visit 6 will be at the clinic (if a blood draw is required) or will be conducted by telephone (if no blood draw is required) • Clarified IDMC review and recommendation to begin enrollment in Part B • Clarified Regeneron PK data review to confirm Part B dose and the addition of PK dosing adjustment rules • In Part B, stratification from country to region was changed • The gestational age categories as ≤31 weeks, 6 days GA or from 32 weeks, 0 days to 35 weeks, 6 days GA was clarified • The Part B treatment arms were clarified • In Part B, monitoring of the subject to 1 hour after dosing was clarified • In Part B, the duration of subject participation was changed • Changed the frequency of site contacts to weekly Clarified information concerning the Medical Information Packet by changing the name, by updating the contents of the Medical Information Packet (including removal of swab samples), and by defining the time to the associated follow-up visit • Added the description of the Safety Monitoring Team • Described the unscheduled visit for potential respiratory illness, including assessments • In Part B, the statement regarding availability of study staff by telephone 7 days a week was removed • study stopping rules were added • The countries and regions where study sites may be located were clarified • the study population age was clarified.

•Extended the Part B Post-Dose Follow-up Period from 93 days to 180 days • Added a section describing the benefit/risk assessment • Updated assessments to be performed at unscheduled visits • Updated the study stopping rules • Clarified that some standard of care treatments are permitted • Included axillary temperature equivalents to rectal temperature • Added a section describing the sponsor’s reporting responsibilities • Made minor clarifications to the text

The following changes were requested by the Pediatric Committee of the European Medicines Agency (PDC) as part of the Pediatric Investigation Plan (PIP) negotiations: • Added 2 secondary endpoints to Part B − Rationale: The addition of secondary endpoints for PK and Antidrug antibody (ADA) titers were to align with agreed PIP. • Added RSV LRTI Analysis and Alternative LRTI Definition Analysis as sensitivity analyses to the primary endpoint − Rationale: To align with the EU protocol and requested by Pediatric Committee (PDCO), respectively The following changes were requested by the Argentinian regulatory authority (ANMAT): • Clarified language in Exclusion Criteria for the Mother − Rationale: To add that infants of mothers who are either 16 years or younger or of an age where they cannot legally provide informed consent in their state/country are excluded. The following additional changes were made: • updated the scientific/medical monitor • clarified the window in Part B during which assessment of primary and secondary clinical endpoints will occur − Rationale: Although no change of the actual window in which primary and secondary endpoint acquisition was made, the change was to clarify that the windows end at different times during the study period. Clarify that subjects who completed Day 150 may be eligible to enroll in a subsequent extension study • Changed the screening period in Part B from 14 to 28 days − Rationale: Based on feedback from study PIs that a longer period from screening to enrollment is required if infants are enrolled in the hospital or neonatal intensive care unit but were enrolled in the study clinic. Removed throat swab collection − Rationale: Based on feedback that because of high sensitivity of RT-PCR for diagnosis of RSV, a throat swab in addition to nose swab is not necessary and could be challenging to collect in some preterm infants.

Revised the approach of handling of missing data in the primary efficacy analysis as discussed with the United States Food and Drug Administration (US FDA). Specifically, the new statistical approach was directly imputed missing data at Day 150 visit. For those subjects who died prior to Day 150 and the deaths were adjudicated to be RSV related, their missing primary endpoint was imputed as “event occurred”. Remaining early termination subjects (including non-RSV deaths) across all treatment groups, was imputed to the average placebo score (estimated placebo event rate). The approach to estimating the placebo event rate was the Kaplan-Meier estimate at Day 150. • Added power calculation for anticipated smaller sample size - Rationale: To understand the impact of the reduced study enrollment on the primary efficacy analysis. • Revised statistical approach to controlling the overall type I error for the primary efficacy analysis. Rationale: To retain sufficient study power for the primary efficacy endpoint given the reduced sample size. To clarify timing of statistical analyses. Specifically, the efficacy analysis was conducted following completion of the 150-day efficacy assessment period by all subjects. This represented the final analysis of all efficacy endpoints. Rationale: Due to the time sensitivity of future study initiation, this supported timely initiation of such studies and avoid long delays waiting for the start of a RSV season.