This pilot study aims to optimize the sample processing techniques for collecting and analyzing skin biopsies obtained from individuals routinely vaccinated against yellow fever. By addressing logistical challenges early, risks will be mitigated and the main study's efficiency will be enhanced.

In order to do so, 15 participants will be identified at the ITM polyclinic after receiving a yellow fever vaccination. Participants will be invited to one study visit at the ITM Clinical Trial Site (CTS) 28 days after vaccination to collect blood samples, skin punch biopsies and a minimally-invasive microbiopsy at the site of vaccination. To investigate the ideal location for the skin punch biopsies, participants will be divided in 3 groups; in the first 5 participants the biopsies will be taken directly on the vaccination site, for the next 5 participants, the biopsies will be taken at +/- 3 cm distance from the vaccination site, and for the last five participants, the biopsies will be taken at +/- 10 cm distance from the vaccination site.

Inclusion Criteria:

• BMI lower than 30kg/m²
• Able and willing to provide written informed consent
• Having received a yellow fever vaccination 28 days (+2 or -2 days) ago

Exclusion Criteria:

• Any confirmed or suspected immunosuppressive or immunodeficient state (incl. cancer); asplenia; recurrent severe infections and use of immunosuppressant medication within the last 6 months prior to recruitment, except topical or short-term oral steroids
• History of bleeding disorder (e.g., factor deficiency, coagulopathy or platelet disorder), or prior history of significant bleeding or bruising following venipuncture
• Any other significant disease, disorder, planned surgery, or finding which may significantly affect the ability of the volunteer to participate in the study
• Tendency to keloid (scar) formation in response to skin damage

Skin diseases at the biopsy or vaccination site