 Travelers n 30, 15 taking low-dose methotrexate MTX, 15 healthy controls HC, seeking travel advice in one of the following Swiss Travel Centers Aarau, Basel, Bern, Geneva, Lausanne, Zurich and who have an indication for yellow fever YF vaccination according to the Swiss Federal Office of Public Healths vaccination recommendations are invited to participate in this study. After signing the consent form i YF viremia and ii anti-YF antibody production in patients taking low-dose MTX and HC will be compared after YF vaccination. It will be analyzed whether the percentage of people with protective antibodies differs between the two groups and iii vaccine side effects will be compared between the groups. 