Simultaneous vaccination in Japanese travelers

BACKGROUND: Simultaneous vaccination is not common in Japan because there is little information available on its effects. Some people are quite concerned about the possibility of adverse reactions due to simultaneous vaccination. The objective of this study was to evaluate whether the frequency and severity of adverse effects are increased by simultaneous vaccination in comparison to single vaccination. METHOD: A retrospective observational study was conducted in 399 asymptomatic travelers who visited the travel clinic during the period January-July 2005. One hundred forty-two participants were given a single vaccination, 257 participants were given simultaneous vaccination. Travel-specific vaccinations were for hepatitis A, hepatitis B, tetanus, rabies and Japanese encephalitis, and routine vaccines were for diphtheria+tetanus, measles, mumps and oral polio vaccine. To evaluate adverse effects, travelers were asked to complete a prepared questionnaire after vaccination. RESULTS: Adverse effects were reported by 26.3% of travelers, with 21.8% reporting local reactions and 4.5% reporting systemic reactions. The simultaneous vaccination group reported significantly more frequent adverse effects than those reported by the single vaccination group. Particularly, tetanus vaccination was shown to significantly raise the risk of adverse effects (P<0.001). However, no serious adverse effects were reported. CONCLUSIONS: Simultaneous vaccination was feasible for Japanese travelers because most problems were generally minor and related to local reactions at the sites of injections. Provision of a simultaneous vaccination schedule should motivate more Japanese travelers to obtain immunizations and thereby reduce the risk of vaccine-preventable diseases.