Burden of severe illness associated with laboratory-confirmed influenza in adults aged 50-64 years, 2010/11 to 2016/17


 
 
 Understanding the burden of influenza is necessary to optimize recommendations for influenza vaccination. We describe the epidemiology of severe influenza in 50-64-year-old residents of metropolitan Toronto and Peel region, Canada, over seven influenza seasons.
 
 
 
 Prospective population-based surveillance for hospitalization associated with laboratory-confirmed influenza was conducted from 9/2010-8/2017. Conditions increasing risk of influenza complications were as defined by Canada’s National Advisory Committee on Immunization. Age-specific prevalence of medical conditions was estimated using Ontario health administrative data. Population rates were estimated using Statistics Canada data.
 
 
 
 Over 7 seasons, 1,228 hospitalizations occurred in patients aged 50-64 years: 40% due to A(H3N2), 30% A(H1N1), and 22% influenza B. The average annual hospitalization rate was 15.6, 20.9 and 33.2/100,000 in 50-54, 55-59, and 60-64-year-olds; average annual mortality was 0.9/100,000. Overall, 33% of patients had received current season influenza vaccine; 963 (86%) had ≥1 underlying condition increasing influenza complication risk. The most common underlying medical conditions were chronic lung disease (38%) and diabetes mellitus (31%); 25% of patients were immunocompromised. The average annual hospitalization rate was 6.1/100,000 in those without and 41/100,000 in those with any underlying condition, and highest in those with renal disease or immunocompromise (138 and 281/100,000 respectively). The case fatality rate in hospitalized patients was 4.4%; median length of stay was 4 days (IQR 2-8).
 
 
 
 The burden of severe influenza in 50-64-year-olds remains significant despite our universal publicly funded vaccination program. These data may assist in improving estimates of the cost-effectiveness of new strategies to reduce this burden.
