Characteristics of Responders
Of the 7986 invited subjects, 4173 (52.3%) responded (Figure 1). Basic characteristics of the responders are given in Table 1. Their median age was 53 years and a slight dominance of women was observed (52.2%). Among the 3967 responders included in the final analysis, 445 reported having a physician-diagnosed asthma yielding asthma prevalence of 11.2% (95% CI 10.3–12.2%). More in detail, 192 of 1898 men (10.1%; 95% CI 8.8–11.6%) and 253 of 2069 women (12.2%; 95% CI 10.9–13.7%) reported having asthma (p = 0.04) and the median age at asthma diagnosis was 21 (IQR 7–43) years in men and 29 (IQR 15–45) years in women (p = 0.03). Of the responders, 47.5% were either current or ex-smokers. Physician-diagnosed allergic rhinitis was reported by 7.2% of the subjects due to pollens only, by 3.1% due to other airborne allergens only and by 7.5% due to both of these, constituting 17.8% overall prevalence of allergic rhinitis (Table 1). Allergic conjunctivitis was reported by 11.7% of the subjects. The non-responder analysis of the study is published elsewhere.
Characteristics of Responders With Asthma According to Presence of Allergic Rhinitis
Of the 445 responders with physician-diagnosed asthma, 230 (51.7%) had also allergic rhinitis (asthma considered as allergic) while 215 (48.3%) did not have allergic rhinitis (asthma considered as non-allergic) (Table 2). Non-allergic asthmatics were slightly older (58 vs 44 years, p < 0.001) and had a clearly older age at diagnosis of asthma (35 vs 19 years, p < 0.001) as compared to allergic asthmatics. In addition, subjects with non-allergic asthma had also slightly higher BMI (p = 0.046) and were more frequently ex-smokers compared to allergic asthmatics (p = 0.034) (Table 2).
Age-specific Incidence of Allergic and Non-allergic Asthma and Their Proportions
Figure 2 shows incidence of allergic and non-allergic asthma in different age groups. A steady decline was observed in the incidence of allergic asthma with advancing age. The incidence of allergic asthma was highest in the youngest age group of 0–9 years (1.8/1000/year) and lowest in the age group of 50–59 years (0.6/1000/year). On the contrary, the incidence of non-allergic asthma was quite low and steady during childhood and early adulthood (about 0.7/1000/year) but it increased markedly after middle age and was highest (2.4/1000/year) in the age group of 50–59 year olds. Overall, the age-specific variation in incidence seemed to be higher for non-allergic than allergic asthma.
Incidence of new asthma diagnoses/1000 person-years divided into allergic (subjects with allergic rhinitis) and non-allergic (subjects without allergic rhinitis) cases in different age groups
Figure 3 shows proportions of allergic and non-allergic asthma among new diagnoses of asthma in different age groups. In the age groups 0–9, 10–19, 20–29, 30–39, 40–49, 50–59 and 60–69 years, 70.4, 62.0, 58.3, 52.5, 37.7, 19.2 and 33.3% of the new asthma cases, respectively, were classified as allergic. More than 60% of the subjects with asthma diagnosed in childhood (< 18 years) reported having allergic rhinitis and thus were considered to have allergic asthma. Therefore, allergic asthma was the dominant phenotype as compared to non-allergic asthma until late twenties while non-allergic asthma became the dominant phenotype around the age of 40 and up to 80% of new cases of asthma were non-allergic in the older ages.
Relative proportions of allergic (subjects with allergic rhinitis) and non-allergic (subjects without allergic rhinitis) cases of new asthma diagnoses in different age groups institutional affiliations.
Sensitivity Analysis and Controlling for Possible Cohort Effect
To assess if the results depend on the definition of allergic asthma, a sensitivity analysis was performed and the results were quite similar when allergic conjunctivitis was included in the definition of allergy. Accordingly, among responders with asthma diagnosed at ages 0–9, 10–19, 20–29, 30–39, 40–49, 50–59 and 60–69 years, 72.4, 66.2, 63.3, 57.6, 47.8, 26.9 and 44.4% respectively, had either allergic rhinitis or allergic conjunctivitis or both (see Additional file 1: Figure S1). Incidence of non-allergic asthma was lowest in the younger ages (0.6/1000/year) and highest in the older age groups (2.2/1000/year in 50–59 years old) (see Additional file 2: Figure S2).
Relative proportions of allergic (subjects with either allergic rhinitis or allergic conjunctivitis or both) and non-allergic (subjects without allergic rhinitis or allergic conjunctivitis) cases of new asthma diagnoses in different age groups.
Incidence of new asthma diagnoses/1000 person-years divided into allergic (subjects with either allergic rhinitis or allergic conjunctivitis or both) and non-allergic (subjects without allergic rhinitis or allergic conjunctivitis) cases in different age groups.
We found that in our sample prevalence of allergic rhinitis was lower in older age groups compared to younger age groups (prevalence of allergic rhinitis was 23.4, 27.2, 23.5, 15.6 and 10.5% in the age groups 20–29, 30–39, 40–49, 50–59 and 60–69 years, respectively, p = 0.036). To assess if the dominance of allergic cases among subjects diagnosed with asthma before the age of 40 years is dependent on the age cohort analyzed, we compared the proportions of allergic asthmatics among subjects diagnosed with asthma before the age of 40 years between three groups according to their current age: currently below 40 years of age, 40–60 years and over 60 years. Proportion of allergic asthma among asthma diagnosed before the age of 40 years was not statistically significantly different between the age groups (64% in subjects <40 years of age, 64% in subjects 40–60 years and 54% in subjects ≥60 years, p = 0.247).
BMC Pulm Med. 2020;20(9) © 2020 BioMed Central, Ltd.