BackgroundWhile the high burden of illness caused by seasonal influenza in children and the elderly is well recognize, less is known about the burden in adults 50â64âyears of age. The lack of data for this age group is a key challenge in evaluating the costâeffectiveness of immunization programs. We aimed to assess influenzaâassociated hospitalization and mortality rates and case fatality rates for hospitalized cases among adults aged 50â64âyears.MethodsThis rapid review was conducted according to the PRISMA; we searched MEDLINE, EMBASE, Cochrane, Web of Science, and grey literature for articles and reports published since 2010. Studies reporting rates of hospitalization and/or mortality associated with laboratoryâconfirmed influenza among adults 50â64 or 45â64âyears of age for the 2010â11 through 2019â20 seasons were included.ResultsTwenty studies from 13 countries were reviewed. Reported rates of hospitalization associated with laboratoryâconfirmed influenza were 5.7 to 112.8 per 100,000. Rates tended to be higher in the 2015â2019 compared with the 2010â2014 seasons and were higher in studies reporting data from highâincome versus low and middleâincome countries. Mortality rates were reported in only one study, with rates ranging from 0.8 to 3.5 per 100,000 in four different seasons. The case fatality rate among those hospitalized with influenza, as reported by populationâbased studies, ranged from 1.3% to 5.6%.ConclusionsSeasonal influenza imposes a significant burden of morbidity on adults 50â64âyears of age but with high heterogeneity across seasons and geographic regions. Ongoing surveillance is required to improve estimates of burden to better inform influenza vaccination and other public health policies.
