• Evaluate serum concentrations of MEDI8897 and palivizumab [ Time Frame: 360 days post dose ]
  Individual MEDI8897 and palivizumab serum concentration data will be tabulated by treatment group along with descriptive statistics. PK parameters will be estimated using non compartmental analysis, if data permit.
• Incidence of anti-drug antibody to MEDI8897 and palivizumab in serum [ Time Frame: 360 days post dose ]
  The incidence of ADA to MEDI8897 and palivizumab will be assessed and summarized by number and percentage of subjects that are ADA positive by treatment group.
• Assess the descriptive efficacy of MEDI8897 in reducing medically attended LRTI incidence and hospitalization due to Reverse Transcriptase Chain Reaction (RT-PCR) confirmed RSV [ Time Frame: 150 days post dose ]
  Incidence of medically attended LRTI (inpatient and outpatient) due to RT-PCR-confirmed RSV through 150 days after Dose 1 for season 1 and season 2

• Drug: MEDI8897
  Anti-RSV monoclonal antibody with an extended half-life
• Drug: Palivizumab
  Approved anti-RSV monoclonal antibody

• Experimental: MEDI8897
  anti-RSV monoclonal antibody with an extended half-life
  Intervention: Drug: MEDI8897
• Active Comparator: Palivizumab
  anti-RSV monoclonal antibody
  Intervention: Drug: Palivizumab

Inclusion criteria

1. For the preterm cohort (excluding subjects with CLD or hemodynamically significant CHD): preterm infants in their first year of life and born ≤ 35 weeks 0 days GA eligible to receive palivizumab in accordance with national or local guidelines, including those with:

   1. Uncomplicated small atrial or ventricular septal defects or patent ductus arteriosus, or
   2. Aortic stenosis, pulmonic stenosis, or coarctation of the aorta alone

2. For the CLD/CHD cohort:

   1. Subjects with CLD - infants in their first year of life and a diagnosis of CLD of prematurity requiring medical intervention/management (ie, supplemental oxygen, bronchodilators, or diuretics) within the 6 months prior to randomization
   2. Subjects with CHD - infants in their first year of life and documented, hemodynamically significant CHD (must be unoperated or partially corrected CHD) Note: Infants with hemodynamically significant acyanotic cardiac lesions must have pulmonary hypertension (≥ 40 mmHg measured pressure in the pulmonary artery) or the need for daily medication to manage CHD
3. Infants who are entering their first RSV season at the time of screening
4. Written informed consent and any locally required authorization (eg, Health Insurance Portability and Accountability Act in the USA, EU Data Privacy Directive in the EU) obtained from the subject's parent(s)/legal representative(s) prior to performing any protocol-related procedures, including screening evaluations
5. Subject's parent(s)/legal representative(s) able to understand and comply with the requirements of the protocol including follow-up and illness visits as judged by the investigator
6. Subject is available to complete the follow-up period, which will be 1 year after Season 1/ Dose 1 for subjects without CLD/CHD, or 1 year after Season 2/Dose 1 (or last replacement dose as applicable for CHD) for subjects with CLD/CHD

Exclusion criteria

1. Any fever (≥ 100.4°F [≥ 38.0°C], regardless of route) or acute illness within 7 days prior to randomization
2. Any history of LRTI or active LRTI prior to, or at the time of, randomization
3. Known history of RSV infection or active RSV infection prior to, or at the time of, randomization
4. Hospitalization at the time of randomization, unless discharge is expected within the 7 days after randomization
5. Requirement for mechanical ventilation, extracorporeal membrane oxygenation, CPAP, or other mechanical respiratory or cardiac support at the time of randomization
6. Anticipated cardiac surgery within 2 weeks after randomization
7. Anticipated survival of < 6 months after randomization
8. Receipt of any investigational drug
9. Known renal impairment
10. Known hepatic dysfunction including known or suspected active or chronic hepatitis infection
11. Clinically significant congenital anomaly of the respiratory tract
12. Chronic seizure, or evolving or unstable neurologic disorder
13. Prior history of a suspected or actual acute life-threatening event
14. Known immunodeficiency, including human immunodeficiency virus (HIV)
15. Mother with HIV infection (unless the child has been proven to be not infected)
16. Any known allergy, including to immunoglobulin products, or history of allergic reaction
17. Receipt of palivizumab or other RSV mAb or any RSV vaccine, including maternal RSV vaccination
18. Receipt of any monoclonal or polyclonal antibody (for example, hepatitis B immune globulin, intravenous immunoglobulin) or anticipated use during the study
19. Any condition that, in the opinion of the investigator, would interfere with evaluation of the study drug or interpretation of subject safety or study results
20. Concurrent enrollment in another interventional study
21. Children of employees of the sponsor, clinical study site, or any other individuals involved with the conduct of the study, or immediate family members of such individuals