Inclusion Criteria:

• Subjects who participated in Study 223 will be eligible for participation in this study if:
• they understand the nature of the study, agree to its provisions and provide written informed consent
• they received the first booster vaccination with FSME-IMMUN 0.5ml during the course of study 223
• blood was drawn after their first booster vaccination in Study 223.

Exclusion Criteria:

Subjects will be excluded from participation in this study if they:

• received any TBE vaccination since their first booster vaccination with FSME-IMMUN 0.5ml
• have a history of infection with or vaccination against other flaviviruses (e.g. dengue fever, yellow fever, Japanese B-encephalitis) since their first booster vaccination with FSME-IMMUN 0.5ml
• are known to be HIV positive (a special HIV test is not required for the purpose of the study) since their first booster vaccination with FSME-IMMUN 0.5ml
• have a known or suspected problem with drug or alcohol abuse (> 4 liters wine / week or equivalent level of other alcoholic beverages).

Subjects will not be eligible for booster vaccination if they:

• are not clinically healthy, (i. e. the physician would have reservations vaccinating with FSME-IMMUN 0.5ml outside the scope of a clinical trial)
• suffer from a disease (e.g. autoimmune disease) or are undergoing a form of treatment (e.g. systemic corticosteroids) that can be expected to influence immunological functions
• are females of childbearing potential and are pregnant or breastfeeding before the booster vaccination (positive pregnancy test result at the medical examination before the booster vaccination)
• have shown an allergic reaction to one of the components of the vaccine since their first booster vaccination in Study 223
• are simultaneously participating in another clinical trial including administration of an investigational product within six weeks prior to the booster vaccination until the end of the study
• do not agree to keep a Subject Diary.
• Subjects who meet the eligibility criteria, but have a febrile illness (body temperature >= 38.0°C, measured orally) at the scheduled time of vaccination, will not be vaccinated until their body temperature returns to normal.
• Subjects who have been administered any vaccination within 2 weeks prior to the booster vaccination will not be vaccinated until an interval of two weeks has passed.
• If subjects have received antipyretics within 4 hours prior to the scheduled time of vaccination, the vaccination should be performed at a later time.
• If a subject had a tick-bite within 4 weeks of the scheduled booster vaccination, the vaccination shall be delayed such that an interval of 4 weeks has passed since the tick-bite in order to avoid any interference with diagnostic assays in the event of a TBE infection.
• If a subject has donated blood or plasma or has received a blood transfusion or immunoglobulins within 4 weeks of the scheduled booster vaccination, the vaccination shall be delayed until an interval of 4 weeks has passed.