The administration of a live attenuated varicella vaccine was licensed in Taiwan in 1997. Both Merck and GlaxoSmithKline, pharmaceutical companies, provided the varicella vaccines in Taiwan. In 2004 the varicella vaccine was incorporated into Taiwan's national immunization program, and all 1-year-old children could receive the varicella vaccine for free. The varicella coverage rate for 1-year-old children was 94% for the 2003 birth cohort, 95% for the 2004 cohort, and 97% for the 2005, 2006, and 2007 birth cohorts according to Taiwan's National Immunization Information System which was developed in the early 1990s . When a child received a vaccine, his or her vaccination would not only be recorded in a child's vaccination handbook but would also be reported to the National Immunization Information System.
Taiwan's National Health Insurance (NHI) covered most of the health care costs for 98% of its population. Taiwan's NHI database includes health care data collected from over 95% of the hospitals in Taiwan for more than 98% of the population receiving health care. From this database, hospitalization and outpatient healthcare records were collected from 2000 to 2008, and International Classifications of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes related to varicella or chickenpox were analyzed for all the population. The age-specific annual incidence and hospitalization rate were calculated. For calculating the annual population-based incidence, the annual incidence was calculated by dividing the number of varicella-related diseases by the population, which was obtained from Department of Statistics, Ministry of the Interior, Taiwan from 2000 to 2008 .
ICD-9-CM codes for chickenpox or varicella include the following: 052 chickenpox; 052.0 post-varicella encephalitis, post-chickenpox encephalitis; 052.1 varicella (hemorrhagic) pneumonitis; 052.7 with other specified complications; 052.8 with unspecified complication; 052.9 varicella without mention of complication.
For complications, we define varicella or chickenpox patients to have complications in cases where the patients had both ICD-9-CM codes for varicella or chickenpox (052 and the other related codes) and also ICD-9-CM codes for varicella-related complications which include the following: central nervous system including 320 meningitis, 322 cerebellitis, 323 encephalitis, 348 encephalopathy, 351 facial palsy, 331.81 Reye's syndrome, 780.3 febrile convulsion/seizure; skin and soft tissue including 680-686 cellulites and abscess, 035 erysipela, 728 pyomyositis and necrotizing fasciitis, 373 & 376.01 blepharitis, 034 & 041 scarlet fever and streptococcal or staphylococcal infection; skeletal system including 711 arthritis, 730-733 osteomyelitis; lower respiratory tract infection including 480-487 pneumonia, 510-519 pneumonitis, 466 & 490 bronchitis; hematological system- 287 thrombocytopenia, 283 & 285 anemia, 288 neutropenia; 038 & 790 & 995.91-995.92 for sepsis and bacteremia; 040-041 for other bacterial infection; 422 cardiomyopathy, 425 myocarditis; 070.5 & 070.9 & 573 for hepatitis.
In univariate analysis, categorical variables were compared with chi-square or Fisher's exact test; continuous variables were analyzed with Student's t test. The difference of annual incidence among various age groups, the difference of annual incidences in different years, and the difference in seasonal distribution were measured with appropriate χ2 test. A p value of < 0.05 was considered to be statistically significant. All statistical analyses were performed with SAS software, version 9.0.