BACKGROUND: Allergic asthma is associated with increased risk of respiratory tract infections and exacerbations. It remains unclear whether this susceptibility is conditioned by seasonal or by perennial allergy.

AIM: To investigate perennial allergy compared with seasonal allergy as a risk factor for lower respiratory tract infections and exacerbations in asthma and whether this risk can be reduced by allergen immunotherapy (AIT).

METHODOLOGY: This is a prospective register-based nationwide study of 18-44-year-olds treated with AIT during1995-2014. Based on the type of AIT and use of anti-asthmatic drugs, patients were subdivided into two groups: perennial allergic asthma (PAA) versus seasonal allergic asthma (SAA). Data on antibiotics against lower respiratory tract infections (LRTI) and oral corticosteroids for exacerbations were analyzed before starting AIT (baseline) and three years after completing AIT (follow-up).

RESULTS: We identified 2688 patients with asthma treated with AIT, among whom, 1249 had PAA and 1439 had SAA. At baseline, patients with SAA had more exacerbations, 23.8%, respectively, 16.5% p=<0.001 but there were no differences in LRTI. During the three-year follow-up, we observed a highly significant reduction of exacerbations with an average decrease of 57% in PAA and 74% in SAA. We also observed a significant reduction of LRTI in both PAA and SAA: 17% and 20% decrease, respectively.

CONCLUSION: AIT effectively reduced the risk of exacerbations and lower respiratory tract infections in both seasonal- and perennial allergic asthma. Perennial allergy is seemingly not a stronger risk factor for respiratory infections and exacerbations than is seasonal allergy.