Case-control study of allergic reactions to Japanese encephalitis vaccine.

Japanese encephalitis (JE) vaccine is widely used in Asia for childhood immunizations, but the vaccine is also used for travellers to Asia from other parts of the world. In Denmark, more than 400,000 doses have been distributed from Statens Serum Institut since 1982. In 1989, the first allergic mucocutaneous reactions after JE vaccination were registered in Denmark and, although the number of reactions have decreased since 1992, reactions are still observed. No explanation of these reactions have been found. The present case-control study, including 49 travellers with allergic reactions and 148 travellers without similar reactions after JE vaccination was performed in order to clarify any possible risk factors. About one third of the adverse reactions to the vaccine could be attributed to an allergic predisposition in the vaccinees. The main risk factors were young age, female gender and previous allergic skin reactions or hayfever. The study also indicated that cases more often reacted to nickel and more often had severe edema after mosquito or other insect bites. Hormone intake was more often spontaneously reported by females in the case group. Accordingly, information on any history of allergy in young adults should be given before JE vaccination, the vaccination should be carried out more than a week before departure and antihistamine treatment should be available if a reaction occurs.