Asthma severity is influenced by complex immunologic and environmental factors. While allergic asthma is linked to increased susceptibility to respiratory infections, the combined role of allergy and antibiotic-treated infections in progression to severe asthma has not been fully evaluated.

To evaluate whether allergic asthma and recurrent respiratory infections (RRI) requiring antibiotics are associated with increased risk of developing severe asthma.

We conducted a registry-based cohort study using Swedish national registry data. Adults with mild-to-moderate asthma were identified in 2014 (baseline) based on prescription records and absence of severe disease indicators. During a two-year exposure window (2015–2016), RRI was defined as ≥ 2 antibiotic prescriptions for lower respiratory tract infections. The outcome was development of severe asthma during 2017–2019, based on ERS/ATS treatment criteria. Allergic asthma was defined by ≥ 2 prescriptions for anti-allergic medications at baseline.

Among 113,393 patients, 24,692 (21.8%) had allergic asthma. RRI occurred more frequently in allergic versus non-allergic asthma (7.5% vs. 5.9%, p < 0.001). A total of 869 patients (0.77%) developed severe asthma. Incidence was higher in those with RRI and highest among patients with both allergic asthma and RRI (2.0%), corresponding to a relative risk of 3.47 (95% CI: 2.49–4.83) versus patients with neither exposure. Results were consistent after adjustment for age, sex and comorbidities.

Allergic asthma and antibiotic-treated respiratory infections were independent and additive predictors of severe asthma progression. These findings support a clinically actionable risk profile and may inform targeted preventive strategies in asthma management.