The present invention is directed to an artificial nucleic acid and to a polypeptide suitable for use in the treatment or prophylaxis of an infection with a flavivirus, in particular an infection with yellow fever virus or with dengue virus, or of a disorder related to such an infection. The present invention is also directed to a composition, preferably an immunogenic composition, comprising the artificial nucleic acid or the inventive polypeptide. In particular, the present invention concerns an immunogenic composition against a flavivirus, such as yellow fever virus or dengue virus. Further, the invention concerns a kit, particularly a kit of parts, comprising the artificial nucleic acid, polypeptide or (immunogenic) composition. The invention is further directed to a method of treating or preventing a disorder or a disease, first and second medical uses of the artificial nucleic acid, polypeptide, composition, in particular the first and second medical uses of the immunogenic composition according to the invention.