Objective : To know how to conduct good pharmacoepidemiology studies using hospital-based database in Japan.
Methods : Medical records during 15 months January 1996 and March 1997 in the University of Tokyo Hospital Information System (HIS) are examined know whether it is possible to conduct pharmacoepidemiology studies similar to previous studies on asthma drugs (Spitzer et al, 1992) and calcium antagonists (Psaty et al, 1995). To know the stability of population covered by HIS, the following two intervals are calculated for ambulatory patients with asthma and hypertension ; 1) average intervals of successive two outpatient visits and 2) intervals between the last day of outpatient visit and the last day of observation.
Results : The size of possible pharmacoepidemiology studies attainable using HIS is judged to be more than 5% of previous studies in Canada and America. Average intervals of successive two outpatient visits are estimated to be 30 days or less for 59% of 693 asthmatics and 77% of 2842 hypertensives. For 48% of asthmatics and 71% of hypertensives, intervals between the last day of outpatient visit and the last day of observation are estimated to be 30 days or less.
Discussion : To attain a size appropriate for pharmacoepidemiology study, researchers must cooperate across hospitals. Although a patient can visit any hospital anywhere under Japanese comprehensive medical care plan, it seems that patients tend to become to visit one particular hospital. However, additional information on medical care in other hospitals is needed for each study subject.
Conclusion : Japanese hospital-based database is suitable for pharmacoepidemiology studies as a record during a long time period is usually available for a large fraction of patients with a particular disease. The study may be free from some of biases closely associated with referral processes known to occur in hospital case-control studies. A design of case-control study selecting patients with long medical records across 5-10 hospitals is probably the most promising when using Japanese hospital-based databases.

 In Horokanai town, Hokkaido, the policy of full subsidies for voluntary vaccinations against influenza, haemophilus influenzae type b (Hib), varicella, mumps, pneumococcal for children and human papillomavirus (HPV) was introduced between 2008 and 2010. A campaign for community education about vaccination was initiated.
 Vaccination coverage improved after the subsidy as follows : influenza vaccination increased from 57.4% to 60.1%, Hib from 2.9% to 52.2%, varicella from 0% to 30.0%, mumps from 2.8% to 38.2%, pneumococcal for children from 1.3% to 50.6%, and HPV from 0% to 81.3%.