The risk of lower respiratory tract infection following influenza virus infection: A systematic and narrative review

BACKGROUND: Lower respiratory tract infections (LRTI) are a major cause of morbidity and mortality worldwide, particularly in young children and older adults. Influenza is known to cause severe disease but the risk of developing LRTI following influenza virus infection in various populations has not been systematically reviewed. Such data are important for estimating the impact specific influenza vaccine programs would have on LRTI outcomes in a community. We sought to review the published literature to determine the risk of developing LRTI following an influenza virus infection in individuals of any age. METHODS AND FINDINGS: We conducted a systematic review to identify prospective studies that estimated the incidence of LRTI following laboratory-confirmed influenza virus infection. We searched PubMed, Medline, and Embase databases for relevant literature. We supplemented this search with a narrative review of influenza and LRTI. The systematic review identified two prospective studies that both followed children less than 5 years. We also identified one additional pediatric study from our narrative review meeting the study inclusion criteria. Finally, we summarized recent case-control studies on the etiology of pneumonia in both adults and children. CONCLUSIONS: There is a dearth of prospective studies evaluating the risk of developing LRTI following influenza virus infection. Determining the burden of severe LRTI that is attributable to influenza is necessary to estimate the benefits of influenza vaccine on this important public health outcome. Vaccine probe studies are an efficient way to evaluate these questions and should be encouraged going forward.