MEDI8897

An AE was development of any untoward medical occurrence in a participant or clinical study participant administered medicinal product and which did not necessarily have causal relationship with this treatment. TEAEs were AEs whose onset occurred after receiving nirsevimab through 360 days post second dose. An SAE was any AE that resulted in death, was immediately life-threatening, required inpatient hospitalization or prolongation of existing hospitalization, resulted in persistent or significant disability/incapacity, was a congenital anomaly or birth defect or was an important medical event that might jeopardize the participant or may require medical treatment to prevent 1 of the outcomes listed above. AESIs were based on assessment by investigators following the administration of nirsevimab. An NOCD was a newly diagnosed medical condition of chronic, ongoing nature post administration of study drug.

Incidence of all medically attended LRTI (inpatient and outpatient) due to RT-PCR-confirmed RSV through 150 days after dosing (ie, during a typical 5-month RSV season)

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

An AE was any untoward medical occurrence in a clinical study participant, temporally associated with the use of study treatment, whether or not considered related to the treatment. TEAEs were AEs whose onset occurred after receiving nirsevimab and within 360 days post dose. A TESAE was any AE that resulted in death, was life-threatening, required inpatient hospitalization, resulted in persistent or significant disability/incapacity, was a congenital abnormality, or was medically significant. AESIs were defined as AEs of immediate (type I) hypersensitivity (including anaphylaxis), thrombocytopenia, and immune complex disease following the administration of nirsevimab based on investigator assessment and Medical Dictionary for Regulatory Activities (MedDRA) preferred term (PT) codes. An NOCD was a newly diagnosed medical condition of a chronic, ongoing nature post administration of treatment.

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 any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease temporally associated with the use of study drug, whether or not considered related to the study drug. A serious adverse event (SAE) is an AE resulting in any of the following outcomes or deemed significant for any other reason: death; initial or prolonged inpatient hospitalization; life-threatening situation (immediate risk of dying); persistent or significant disability/incapacity of a participant who received MEDI8897. TEAEs and TESAEs were the events that occurred between administration of study drug (Day 1) and Day 361 that were absent before treatment or that worsened relative to pre-treatment state.

An AESI was one of scientific and medical interest specific to understanding of study product and may have required close monitoring and rapid communication by investigator (that is, within 24 hrs of knowledge of the event) to the sponsor. The AESIs for this study were hepatic function abnormality meeting the definition of Hy's law, hypersensitivity reactions including anaphylaxis, immune complex disease, and thrombocytopenia. Treatment-emergent AESIs were collected from the time of dosing until Day 361 post-dose.

Laboratory abnormalities determined by the investigator to be clinically significant that occurred after study drug dosing through 151 days post-dose that were absent before treatment or that worsened relative to pre-treatment state were reported as TEAEs. Laboratory evaluations (haematology and serum chemistry) of blood samples were performed.

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.