Vaccination against influenza is recommended for subjects at high risk of severe complications if infected with the influenza virus. The effectiveness of influenza vaccination among the elderly has been evaluated in many observational studies but only in a few randomized control trials. However, observational studies have various issues. First, the outcomes should be of high specificity with low nondifferential misclassification, such as laboratory-confirmed influenza or high fever. Differential misclassification between vaccinated and unvaccinated participants may also be a complicating factor due to behavioral differences in the habits of participants with regard to visiting physicians or in the diagnostic processes of physicians. Therefore, preventing differential misclassification by equally precise observation in compared groups seems to be more important than the specificity of outcomes, even if this can be achieved only by techniques with low-specificity outcomes.
 In observational studies of the elderly, vaccinated elderly individuals may be at low risk of influenza infection or death because they are health conscious. Such confounding may lead to an overestimation of the effectiveness of influenza vaccination. To minimize bias and to adjust for confounders, the effectiveness of influenza vaccination should be compared taking into account the following groups/indicators: influenza season and pre-season, well-matched season and mismatched season between vaccine and circulating viruses, severe season and mild season, high-specificity outcomes and low-specificity outcomes. Low effectiveness is to be expected with the latter of each pair. If no difference in effectiveness is observed between the two conditions in each pair, residual confounding may be present and the true underlying risk factors should be sought. For example, elderly individuals with impaired functional status in addition to chronic diseases may be less able to access vaccination. Hence it is necessary to evaluate the effectiveness of vaccination among such elderly individuals and promote vaccination in those people.

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[in Japanese]

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Objectives The aim was to evaluate vaccination coverage among foreign children living in an urban area of Gunma Prefecture, Japan and to examine their parents' views concerning the local immunization services.
Methods A total of 321 foreign children aged 6 to 18 years in five international schools participated in school health examinations and provided vaccination information. Among the parents, 304 completed a questionnaire on their views about the immunization services. Another questionnaire survey was conducted in nursery schools for parents of 4,629 Japanese children aged 0 to 6 years. Of the total, 3,811 (82.3%) responded, and 2,911 questionnaires answered by the parents who had children aged 3 years and older were eligible for the analysis.
Results The study found a vaccination coverage of 86.2% for diphtheria and tetanus toxoids and pertussis (DTP), 86.5% for poliovirus vaccine, and 87.7% for BCG among the foreign children. Of their parents, 84% were born in Brazil. One third of the foreign children vaccinated for DTP, poliovirus and/or BCG had received each vaccine in Japan, while the others children had been vaccinated in their home countries. Among 162 parents with children immunized in Japan, 77% received the necessary information about immunization services from the local municipal office, and 80% had no major problems. However, 15% felt the language barrier. Among the Japanese children, non-vaccination rates for DTP, poliovirus vaccine, and BCG were 18.5%, 9.9%, and 3.5%, respectively. Of the Japanese parents, 85% knew immunization schedules from the municipal office, and 51% asked for night-time and holiday vaccination sites.
Conclusion These results suggest that vaccination coverage for DTP, poliovirus vaccine or BCG is relatively high among foreign children living in Japan. To promote higher rates of vaccination for those residents, however, accessibility of the municipal consultation services in foreign languages should be improved.

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