Introduction Asthma is a complex condition where early respiratory infections are implicated in causation. However, the literature is not always supportive of the paradigm that early respiratory infection increases the risk for later asthma. Here, we test the hypothesis that hospital admission with bronchiolitis or lower respiratory tract infection (LRTI) before age two is associated with increased risk for an asthma admission after two years of age.

Method Details of all paediatric admissions to Scottish hospitals 2000–2013 were obtained. Admissions with bronchiolitis, LRTI and asthma were identified. Each individual had a unique identifier which allowed linkage of admissions.

Results There were 329211 admissions, including 28856 with bronchiolitis, 8558 with LRTI and 14734 with asthma. 2.7% of those with a bronchiolitis admission and 3.8% with a LRTI admission had a later asthma admission (see Table 1). Compared to zero previous bronchiolitis admissions, one admission was associated with a reduced risk for asthma admission (odds ratio [OR] 0.71 [95% CI: 0.65, 0.77] but this risk increased after two bronchiolitis admissions (1.19 [95% CI: 0.99, 1.41]) and after ≥3 admissions (OR 1.52 [95% CI: 1.11, 2.08]). One prior LRTI admission was associated with a borderline increased OR for a later asthma admission (1.12 [95% CI: 0.99, 1.26]) compared to no admissions, and the OR after two previous admissions was 1.49 ([95% CI: 1.05, 2.13]). HDU admission with bronchiolitis or LRTI was not associated with increased risk of later asthma admission.

Conclusion The relationship between early respiratory infections requiring hospitalisation, and later asthma admissions is complex. There is no consistent evidence that hospitalisation with respiratory infections in early life is causally linked to risk of later asthma admissions. However, repeated hospitalizations with infections may be linked to later asthma admissions by reverse causation.