This clinical trial is focused on studying Tick-borne Encephalitis (TBE), a disease caused by a virus transmitted through tick bites. The study will use two vaccines: FSME-IMMUN Vuxen and Encepur. These vaccines are designed to protect against the TBE virus. The purpose of the study is to evaluate how the immune system responds to these vaccines, particularly looking at the body’s T-cell and B-cell responses, which are types of immune cells that help fight infections.

Participants in the study will receive the TBE vaccine through an injection into the muscle. For those receiving the FSME-IMMUN Vuxen vaccine, the schedule includes four doses over eight months. The study will compare the immune responses of people who are healthy and those who have weakened immune systems. Some participants will have been vaccinated before joining the study, while others will start their vaccination during the study.

The study will also explore how long the immune responses last after vaccination or infection with the TBE virus. Additionally, it will investigate whether previous vaccinations against other similar viruses, like Yellow Fever or Japanese Encephalitis, affect the immune response to the TBE virus. The study aims to provide valuable insights into the effectiveness and durability of the immune response generated by the TBE vaccines.