Objectives: The objective was to review the published literature on seasonal influenza to assess the differences between complications and mortality rates for those adults at high risk of influenza complications, including the resource use of those hospitalized with influenza complications.
Methods: A targeted literature review was performed using electronic database keyword searches, specific inclusion criteria, quality rating of the reviewed full-text articles and abstraction of data to present published evidence on the incidence, complication rates and health service use associated with clinical influenza in different adult high-risk groups including those who are aged 65 years and older or those with different chronic underlying medical conditions.
Results: Key findings for incidence rates of clinical influenza were that incidence rates are similar among people with chronic cardiovascular or respiratory comorbidity, and may be higher in those with allogeneic stem cell transplants compared to those with autologous transplants. Rates of hospitalization and/or pneumonia or lower respiratory tract infection for those with chronic conditions or those who are immunocompromised are substantially higher than those in people over age 65 but without additional high-risk factors. A person who is hospitalized and has a laboratory-confirmed influenza diagnosis has a probability of intensive care unit admission of between 11.8-28.6% and of death of between 2.9-14.3%.
Conclusions: These findings indicate that although the burden of influenza varied across high-risk groups, it also varied widely across studies within a single high-risk group. A key finding was that those over 65 years of age but without additional high-risk factors had a low risk of influenza complications. A limitation of the review is that most of the studies of hospitalized patients did not present outcomes data separately by high-risk group and only limited data were identified on rates of hospitalization or lower respiratory tract infection for most high-risk groups. Information about influenza complication rates and resource use, including influenza vaccines, chemoprophylaxis and/or treatment strategies for different high-risk groups, is needed to evaluate new interventions.