This is a randomized, open-label Phase 3 study including children aged >9 months to <17 years and 7 months who have been vaccinated with IXIARO in study IC51-323.

This is a randomized, open-label Phase 3 study including children aged >9 months to <17 years and 7 months who have been vaccinated with IXIARO in the previous study IC51-323.

Inclusion Criteria:

• Children and adolescents who have completed study IC51-323 and received both IXIARO vaccinations according to protocol.
• Children who have received the dose confirmed for their age group.
• Male or female healthy children and adolescents aged ≥9 months to <17 years and 7 months at the time of enrolment into this study.
• Written informed consent by the subject's legal representative(s), according to local requirements, and written informed assent of the subject, if applicable.
• Female subjects: either no childbearing potential or negative pregnancy test (pregnancy test to be performed in female subjects after onset of menarche) at Visits 1, 2 and 2a as stipulated by the protocol. For females after menarche willingness to practice a reliable method of contraception

Exclusion Criteria:

• Vaccination against JE virus (JEV) (except within study IC51-323 and IC51 325), Yellow fever, West Nile virus and Dengue fever at any time prior to or planned during the study.

• History of or clinical manifestation of any Flavivirus disease during IC51-323 or IC51 325.

• Participation in another study with an investigational drug during IC51 323 or IC51 325.

• Planned active or passive immunization within 2 weeks before and 1 week after the IXIARO booster.

• History of or development of any immunodeficiency including post-organ-transplantation after inclusion into IC51-323 or IC51 325.

• History of or development of an autoimmune disease during study IC51-323 or IC51 325.

  • Administration of chronic (defined as more than 14 days) immunosuppressants or other immune-modifying medications started during IC51-323 or IC51 325. (For corticosteroids, this would mean prednisone or equivalent at >0.05 mg/kg/day; topical and inhaled steroids are allowed).

• Acute febrile infection at Visit 2 (only for the Booster Group).

• Pregnancy (positive pregnancy test at Visit 1 and Visit 2), lactation or unreliable contraception in female subjects after onset of menarche.

• Hypersensitivity reactions to IXIARO or adverse events in study IC51-323 requiring withdrawal from further vaccination or anaphylaxis or severe cases of atopy requiring emergency treatment or hospital admission during IC51-323 or IC51 325.

• History of urticaria after hymenoptera envenomation, drugs, physical or other provocations or of idiopathic cause during IC51-323 or IC51 325.

• Known infection with human immunodeficiency virus (HIV), hepatitis B virus (HBV) (measurement of Hepatitis B surface antigen [HBsAg] titers) or hepatitis C virus (HCV).

• Illicit drug use and/or current drug or alcohol addiction.

• Inability or unwillingness by the legal representative(s) and/or the subject (where applicable) to provide informed consent/assent and to abide by the requirements of the study.

• Persons who have been committed to an institution (by a court or by an authority).

Safety Population: subjects who entered into the study and were randomized