The effectiveness of grass, birch, and mugwort pollen allergen immunotherapy over 18 years: A national cohort study in Denmark.

BackgroundLong-term effectiveness of pollen allergen immune therapy (AIT) for allergic rhinitis (AR) has been studied for different durations. We studied effectiveness over 18 years using nationwide register-data for Denmark. MethodsA register-based cohort study using data on filled prescriptions, 1995-2018, Denmark. In a cohort of 1.1 million intranasal corticosteroid spray users (proxy for AR), we matched users treated with grass, birch or mugwort (GBM) AIT 1:2 with non-treated users on baseline year and 24 characteristics in the 3 years prior to baseline. The primary outcome was the odds ratio (OR) of using anti-allergic nasal spray during the pollen season in the treated vs. non-treated group modelled in a logistic generalized estimating equations analysis using an autoregressive correlation structure and adjusted for years since baseline (one year categories). ResultsAmong 7,914 AR patients treated with GBM AIT, the OR of using nasal spray 0-5 years after baseline was reduced when compared with 15,520 non-treated AR individuals (0-2 years, odds ratio (OR) 0.81(0.76-0.85); 3-5 years, OR 0.82(0.77-0.87)), but was close to unity or higher thereafter (6-9 years, OR 0.96(0.91-1.03); 10-18 years, OR 1.15(1.06-1.24). In post-hoc analyses, results were more consistent for those who already had 3 of 3 baseline years of use (0-2 years, OR 0.59 (0.54-0.64); 3-5 years, OR 0.64 (0.59-0.71); 6-9 years, 0.82 (0.75-0.91); 10-18 years, 0.95 (0.84-1.08), and in patients using nasal spray in the last pollen season (0-2 years, OR 0.76 (0.72-0.80); 3-5 years 0.84 (0.78-0.91); 6-9 years, OR 0.94 (0.86-1.03); 10-18 years, 0.90 (0.81-1.00)) as opposed to patients who did not use nasal spray in the last pollen season. The post-hoc findings were likewise more consistent for eye drop and oral antihistamine users (secondary outcomes). ConclusionPatients treated with GBM AIT in routine care to a higher degree stopped using anti-allergic nasal spray 0-5 years after starting the standard three years of therapy. Post-hoc analyses suggested results were more consistent among patients with persistent AR.