To evaluate safety, efficacy and pharmacokinetics of palivizumab in children at the age of 24 months or less with immunocompromised medical conditions.

Inclusion Criteria:

1. Availability of parent or legal guardian who is capable and willing to give written informed consent for his/her newborn, infant or young child to participate this study.

2. Japanese newborn, infant or young child at age of 24 months or less.

3. The subject must meet at least one of the following immunocompromised medical conditions (from [a] to [h]), and must be considered by the investigator to be a suitable candidate to receive prophylactic treatment of palivizumab:

   1. Subject has been diagnosed with combined immunodeficiency (severe combined immunodeficiency, X-linked hyper-immunoglobulin M (IgM) syndrome, etc.), antibody deficiency (X-linked agammaglobulinemia, common variable immunodeficiency, non-X-linked hyper-IgM syndrome, etc.) or other immunodeficiency (Wiskott-Aldrich syndrome, DiGeorge syndrome, etc.) at the time of informed consent, or
   2. Subject has been diagnosed with human immunodeficiency virus infection, or
   3. Subject has been diagnosed with Down syndrome without a current hemodynamically significant congenital heart disease at the time of informed consent (subject must have an experience with persistent respiratory symptom or regular outpatient treatment due to respiratory tract infection prior to current RSV season), or
   4. Subject has a history of post organ transplantation at the time of informed consent, or
   5. Subject has a history of post bone marrow transplantation at the time of informed consent, or
   6. Subject is receiving immunosuppressive chemotherapy at the start of study drug administration, or
   7. Subject is receiving systemic high dose corticosteroid therapy (prednisone equivalents 0.5 mg/kg or more every other day, other than inhaler or topical use) at the start of study drug administration, or
   8. Subject is receiving other immunosuppressive therapy (azathioprine, methotrexate, mizoribine, mycophenolate mofetil, cyclophosphamide, cyclosporine, tacrolimus, cytokine inhibitors, etc.) at the start of study drug administration.

Exclusion Criteria:

1. Subject who meets one of the palivizumab indications already approved in Japan.

   • Subject born at 28 weeks of gestation or less and who is age of 12 months or less at the start of study drug administration.
   • Subject born at 29 - 35 weeks of gestation and who is age of 6 months or less at the start of study drug administration.
   • Subject is age of 24 months or less with a history of bronchopulmonary dysplasia requiring medical management within the 6 months prior to the study drug administration.
   • Subject is age of 24 months or less with a current hemodynamically significant congenital heart disease at the start of study drug administration.

2. Subject requires oxygen supplementation, mechanical ventilation, extracorporeal membrane oxygenation, continuous positive airway pressure or other mechanical respiratory or cardiac support at Screening and at the start of study drug administration.

3. Subject has a current active infection including respiratory syncytial virus infection at Screening and at the start of study drug administration.

4. Subject has a serious concurrent medical condition (hepatic dysfunction, persistent seizure disorder, etc.) except those resulting in an immune deficiency condition or renal failure.

5. Subject has received palivizumab prior to the study drug administration.

6. Subject has received any other investigational agents in the past 3 months or 5 half lives prior to the investigational drug administration (whichever is longer).

7. Subject has a history of an allergic reaction or hypersensitivity to constituents of the study drug.

8. Subject has a history of serious adverse reactions or serious allergic reaction to immunoglobulin products or has a history of hypersensitivity to immunoglobulin products, blood products, or other foreign proteins.

9. Subject whose remaining days of life are expected to be less than one year at the time of informed consent.

10. It will be impossible to collect blood as scheduled from the subject.

11. Subject is considered by the investigator, for any reason, to be an unsuitable candidate for the study.