Background — The cause of rising asthma incidence over time remains unexplained. Examining trends in the age of diagnosis across successive birth cohorts may offer insights into asthma etiology.
Objective — To examine trends in the age at asthma diagnosis and the age and proportion of children hospitalized at first asthma diagnosis in Ontario, Canada.
Methods — Eight consecutive birth cohorts of children (1993-2000) were observed using administrative data from a universal health insurance plan in Ontario, Canada (population 13 million). Trends in the need for hospitalization and age at asthma diagnosis were examined with descriptive and survival analyses.
Results — The records of 1,059,511 children were examined, of whom 201,958 developed asthma in the first 8 years of life, with an average cumulative incidence of 19.1%. Mean age at asthma diagnosis decreased from 4.7 ± 1.5 years in birth year 1993 to 2.6 ± 2.0 years in birth year 2000 (P < .0001), with a higher adjusted risk of asthma diagnosis (hazard ratio, 6.7; 95% CI, 6.5-6.9) in the first 3 years of life for children born after 1996 versus children born in the period 1993 to 1995 (hazard ratio, 1.4; 95% CI, 1.3-1.4). The proportion of children hospitalized at asthma diagnosis stayed stable while the age at first asthma hospitalization decreased over time (P < .0001).
Conclusions — This study demonstrates a significant increase in asthma incidence and a decrease in the age of asthma diagnosis across multiple birth cohorts. Changes in asthma incidence over time are primarily explained by variations in asthma rates in children younger than 3 years.
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