Estimating age-specific influenza-associated asthma morbidity in Ontario, Canada

Background: 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. Methods: 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. Results: 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). Conclusions: 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.