Baseline Assessment of Skin Resident Memory T Cells in Healthy Unvaccinated Participants

In The main GeKoSkimm trial (= A randomized controlled trial to compare the immunogenicity and skin imprinting of intradermal, subcutaneous and intramuscular yellow fever vaccination) the investigators try to understand whether the route of injection (in the muscle, under the skin, or in the skin) when giving the yellow fever vaccine affects the strength of the body's immune response to yellow fever. More in particular the investigators want to know how well the immune responses are retained in the skin, as this is the place where the virus enters the body after a mosquito bite. This will be important for vaccines against infections transmitted via bites of mosquitoes and ticks, such as yellow fever virus. Because this is the first attempt on human samples and only skin samples after vaccination with yellow fever vaccine are available, the investigators want to assess the baseline or background immune response in 'unvaccinated' skin. Therefore, 40 volunteers will be recruited that have not been previously vaccinated against yellow fever and will not receive a yellow fever vaccine during this study. The participant will be requested to provide two skin samples from the upper arm and a blood sample to confirm their vaccination status regarded to yellow fever.

Inclusion Criteria: * BMI \>/=18,5 kg/m2 and ≤35 kg/m2 * Agreement to share and discuss participant's medical history and medical records when relevant with the study team * Able and willing to provide written informed consent * Agreement to refrain from blood donation and other vaccinations during the study Exclusion Criteria: * Participants with a confirmed flavivirus infection in the past * Participants who received a flavivirus vaccination prior to enrolment (e.g. YFV, Japanese encephalitis virus, tick-borne encephalitis virus, Dengue) * Receipt of any vaccine (licensed or experimental) within 30 days prior to enrolment or during the study * Active participation in another interventional clinical study with active substance intake or large medical procedures affecting the study procedures during or 1 month prior to enrolment * Participant has a clinically significant acute illness (this does not include minor illnesses such as diarrhea or mild upper respiratory tract infection) or temperature ≥38.0ºC within 24 hours prior to the visit; inclusion at a later date is permitted at the discretion of the investigator * Any confirmed or suspected immunosuppressive or immunodeficient state (incl. cancer and human immunodeficiency virus/hepatitis B virus infection); asplenia; recurrent severe infections and use of immunosuppressant medication (including antineoplastic and immunomodulating agents or radiotherapy) within the last 6 months prior to enrolment, except topical or short-term oral steroids (\