There is a need to quantify the potential benefits of influenza-focused interventions in reducing asthma morbidity at a population level. This study aims to estimate age-specific annual excess asthma morbidity attributable to influenza in Ontario, Canada.

Weekly counts of hospitalizations, emergency department (ED) visits and outpatient physician office visits for asthma were obtained from health administrative data in Ontario from 2010 to 2015, for ages 0–14, 15–59 and 60+. Asthma morbidity was modelled as a function of influenza A and B activity using linear regression, controlling for seasonal and long-term trend, mean temperature and respiratory syncytial virus. Excess asthma morbidity attributable to influenza was calculated as the difference between full model predictions and model predictions with influenza A and B variables set to 0.

Annually, influenza was associated with the following rates of excess asthma morbidity, per 100,000 people with prevalent asthma: 12.5 hospitalizations for ages 15–59 (95% confidence interval (CI): 1.1–23.5); 35.7 hospitalizations for ages 60+ (95% CI: 3.3–67.1); 114.1 ED visits for ages 15–59 (95% CI: 46.9–181.6); 154.6 ED visits for ages 60+ (95% CI: 86.7–223.3); and 1025.7 outpatient physician office visits for ages 60+ (95% CI: 79.0–1877.3).

Influenza was associated with excess asthma hospitalizations and ED visits for ages 15–59 and 60+ and outpatient physician office visits for ages 60+. Individuals with asthma aged 15–59 and 60+ might be important targets for influenza-focused interventions, to reduce asthma morbidity at the population level.