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).

The ITT population included all participants who were randomized in the study and analyzed according to their randomized treatment group.

A RSV hospitalization is defined as either 1) a respiratory hospitalization with a positive RSV test within 2 days of hospitalization (primary) or 2) new onset of respiratory symptoms in an already hospitalized child, with an objective measure of worsening respiratory status and positive RSV test (nosocomial).

The ITT population included all participants who were randomized in the study and analyzed according to their randomized treatment group.

An adverse event (AE) is any untoward medical occurrence in a participant who received study drug without regard to possibility of causal relationship. An 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 experience (immediate risk of dying); persistent or significant disability/incapacity; congenital anomaly. TEAEs are defined as events present at baseline that worsened in intensity after administration of study drug or events absent at baseline that emerged after administration of study drug.

As-treated population included all randomized participants who received any study drug and analyzed according to the study drug they actually received.

An AESI was one of scientific and medical interest specific to understanding of study drug and may have required close monitoring and rapid communication by investigator to the sponsor. An AESI may be serious or non-serious. A NOCD is a newly diagnosed medical condition that is of a chronic, ongoing nature. It is observed after receiving study drug and is assessed by investigator as medically significant.

As-treated population included all randomized participants who received any study drug and analyzed according to the study drug they actually received.

As-treated population included all randomized participants who received any study drug and analyzed according to the study drug they actually received.

Terminal elimination half-life (t½) is the time required for half of the drug to be eliminated from the serum.

As-treated population included all randomized participants who received any study drug and analyzed according to the study drug they actually received. Participants with sufficient additional pharmacokinetics (PK) samples from unscheduled visits were analysed for this outcome measure.

The number of participants with positive serum antibodies to MEDI8897 are reported.

As-treated population included all randomized participants who received any study drug and analyzed according to the study drug they actually received.