Understanding immune responses to the Yellow Fever (YF) and Japanese Encephalitis (JE) flavivirus vaccines (The FLAVIFLAV Study)

Yellow Fever and Japanese Encephalitis are both serious mosquito-borne viral diseases, and travellers to affected regions are routinely vaccinated against them. Both vaccines use live but weakened viruses, and interestingly, the Japanese Encephalitis vaccine actually contains a small piece of the Yellow Fever virus inside it. When people receive both vaccines, researchers do not yet fully understand whether the order of vaccination affects the strength of the immune response to each. This study will randomly assign healthy participants to receive the two vaccines four months apart — either Yellow Fever first or Japanese Encephalitis first — and will collect blood samples at regular intervals to track how the immune system responds and whether the order makes a difference. This knowledge could help improve vaccination schedules for travellers. This study is for healthy adults aged 18 to 60 who have never had either vaccine before and are willing to have blood drawn on multiple occasions. People with immune conditions, pregnancy, a history of thymus disease, or a severe egg allergy are not eligible.