Primary Endpoint Analysed on Primary Cohort Through 150 Days (N=1490 Participants)

Safety and tolerability of MEDI8897 will be assessed by the occurrence of all treatment-emergent adverse events (TEAEs), treatment-emergent serious adverse events (TESAEs) , adverse events of special interest (AESIs), and new onset chronic diseases (NOCDs)

The determination of medically attended RSV LRTI is based on objective clinical LRTI criteria and RSV test results obtained from analyzing the respiratory secretions using a validated RSV real time reverse transcriptase-polymerase chain reaction (RT-PCR) assay for the detection of RSV A or RSV B subtypes. Criteria for LRTI included documented physical exam findings of rhonchi, rales, crackles, or wheeze and any of the following: increased respiratory rate at rest (for age less than (\<) 2 months: greater than or equal to (\>=) 60 breaths/min; 2-6 months: \>= 50 breaths/min; and for \> 6 months - 2 years, \>= 40 breaths/min), or hypoxemia (in room air - oxygen saturation \< 95% at altitudes less than or equal to (\<=) 1800 meters or \< 92% at altitudes \> 1800 meters), or clinical signs of severe respiratory disease or dehydration secondary to inadequate oral intake due to respiratory distress (need for intravenous fluid).

An adverse event (AE) is defined as events present at baseline that worsened in intensity after administration of investigational products or events absent at baseline that emerged after administration of study drug, for the period extending to 391 (Day 361 ± 30 days) days after the last dose of study drug.