The changing epidemiology of chickenpox in Alberta

BACKGROUND: Varicella vaccine was licensed in Canada in 1998. The province of Alberta introduced a universal publicly funded varicella vaccination program in 2001. PURPOSE: To describe the epidemiology of non-fatal cases of chickenpox for which publicly funded health services were utilized for the period 1986-2002. METHODS: We used the records of Alberta's universal, publicly funded health care insurance system to identify cases of chickenpox for the period 1986-2002. The earliest dated utilization of a health service for which there was an ICD9-CM code of 052.xx or an IC10-CA code of B01.xx was used as the date of illness onset. Denominators for rates were estimated using mid-year population estimates from the Alberta Health Care Insurance Registry. Age-specific rates were estimated for each year. RESULTS: The crude incidence of chickenpox significantly declined over the period 1994-2002, most steeply after the year 2000. The incidence of chickenpox varied by age group and year and there was evidence of age-group-year interaction. Among those aged 5-19 years, chickenpox incidence began to decline prior to vaccine licensure in Canada. Among those aged less than one year and those aged 1-4 years, the incidence increased until 1999 when a decline began. Over the period 0.8% of cases were hospitalized. CONCLUSION: Chickenpox rates began to decline prior to the introduction of the publicly funded vaccination program; however the declines in rates among the youngest age-groups are consistent with a vaccination program effect.