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Background: Asthma and allergic rhinitis are common chronic diseases of childhood.Their prevalence has increased globally in the 20th century, with regionally diverging trends in recent years. Allergic sensitization (AS) is strongly associated with asthma and allergic rhinitis (AR), with similar trends over time and uncertainty of development in recent years. AS increases in childhood until young adulthood. Risk factors for AS are not as well characterized as for asthma and AR, and important aspects of the relation between AS and disease are not known. This includes why some sensitized children do not develop disease and how common this is in a longitudinal perspective. While many studies have assessed AS in the first years of life, fewer have focused on school age, and none have focused on the effect of age at onset of AS. Further, the association between AS and lung function is unknown.

Aims: To assess time trends in prevalence, incidence and factors associated with AS in schoolchildren. Further, to assess the association between AS and asthma, allergic rhinitis and lung function, with special interest in age at onset of sensitization.

Methods: Children in grades 1 and 2 (median age 8 years) in two municipalities in northern Sweden were invited to a questionnaire study of allergic diseases and skin prick tests to common aeroallergens in 1996. The cohort was reassessed at 12 and at 19 years also including spirometry. Identical methods were used to recruit a second and third cohort in 2006 and 2017.

Results: The prevalence of AS at 8 years was 21% (1996), 30% (2006) and 30% (2017). Patterns of sensitization and risk factors for sensitization were similar. In the second cohort, the incidence between 8-12 years of 18% and prevalence of 41% at 12 years were higher than in the first cohort. Compared with later onset, sensitization ≤8 years was more strongly associated with asthma and allergic rhinitis at 19 years. AS was not independently associated with lung function at 19 years.

Conclusions: In northern Sweden, the increase in allergic sensitization at age 8 years between 1996 and 2006 has halted in recent years. The increase between the first two cohorts was seen also at age 12 years. Age at onset of allergic sensitization impacts development of asthma and allergic rhinitis until 19 years. Despite an association with asthma, sensitization does not affect lung function at 19 years.