The epidemiology of allergic sensitization and the relation to asthma and rhinitis

Abstract: Background: Allergic sensitization is the most important risk factor for asthma and rhinitis among children, adolescents and young adults. Less is known about the incidence and remission of allergic sensitization, particularly in older adults. Furthermore, it is not clear if the earlier documented increase in prevalence of allergic sensitization continues. This thesis is focused on prevalence, incidence and remission of allergic sensitization to airborne allergens among adolescents and adults as well as on time trends in prevalence among adults. Furthermore, associated risk factors and the relation of allergic sensitization to asthma and rhinitis were assessed.Methods: In the study of children and adolescents, incidence, remission and prevalence of allergic sensitization were assessed in a cohort study of schoolchildren, aged 7-8 years (y) at baseline. In the studies of adults, incidence and remission of allergic sensitization were assessed in a randomly selected adult population sample in 1994 (n=664) aged 20-69 y, which was followed up in 2004 (n=555). Trends in prevalence of allergic sensitization were assessed by comparing two cross-sectional studies; the cohort from 1994 and another randomly selsected population sample examined in 2009 (n=737). The relation of allergic sensitization to asthma and rhinitis was determined in the adult cohort in 2009. Allergic sensitization was assessed by skin prick test (SPT) with ten common airborne allergens at ages 7-8, 11-12 and 19 y in the cohort of children and in the participants ≤ 60 y in the adult cohorts. Specific IgE to nine airborne allergens was analyzed in the adult cohorts in 2004 and 2009. Risk factors for allergic sensitization and variables defining respiratory disease and symptoms were assessed by questionnaires in the cohort of children and by structured interviews in the adult cohorts.Results: The 10-year cumulative incidence of allergic sensitization among the adults from 1994 to 2004 was 5%, while remission was 32%. In both adult cohorts, the prevalence of allergic sensitization was highest among young adults, aged 20-29 y, 55% and 61% and decreased significantly with increasing age. Among children and adolescents, both incidence and persistence of allergic sensitization were high, and the prevalence of allergic sensitization increased by age from 21% at age 7-8 y to 42% at age 19 y. Multisensitization at age 19 y was strongly associated with early onset of sensitization. The prevalence of sensitization to the major specific allergens birch, timothy, cat and dog as well as multisensitization (from 40% in 1994 to 56% in 2009, p=0.002) increased significantly from 1994 to 2009 among the adults. Sensitization to any allergen increased from 35% to 39%, however not significantly (p=0.13). A family history of allergic rhinitis was strongly and consistently associated with allergic sensitization in all ages. Male sex and urban living were significantly positively and birth order and furry animals at home in childhood were negatively associated with onset of sensitization before the age of 7-8 y, but not with onset of sensitization from 11-12y to 19 y. Young adult age and urban living were significant factors associated with allergic sensitization in adult age. Sensitization to any animal was significantly positively associated with current asthma (OR4.80 (95% CI 2.68-8.60)), whereas both sensitization to any pollen (OR 4.25 (2.55-7.06)) and any animal (OR 3.90 (95% CI 2.31-6.58)) were associated with current allergic rhinitis. The association between allergic sensitization and allergic rhinitis was strongest in young adult age and decreased with increasing age, while asthma was similarly associated with sensitization to any animal across all adult ages. Among asthmatics, the prevalence of allergic sensitization decreased with increasing age of asthma onset.Conclusion: Both incidence and persistence of allergic sensitization were high among children and adolescents explaining the increase in prevalence by increasing age. An inverse pattern with low incidence and high remission of allergic sensitization was seen among adults. The decrease in prevalence of allergic sensitization by increasing adult age might at least partly be explained by normal ageing and not only by an effect of year of birth (cohort effect). The significant increase in prevalence of sensitization to the specific allergens explained the significant increase in multisensitization over 15 years. A family history of allergy was the strongest and the only consistent risk factor for allergic sensitization in all ages. The prevalence of allergic sensitization decreased with increasing age of asthma onset among adult asthmatics.