The Burden of Influenza: a Complex Problem

Influenza has been long-recognized as an important cause of morbidity and mortality in human populations, leading to development and use of vaccines intended to reduce consequent health and economic impacts. Since 1918, four influenza A pandemics have caused substantial numbers of illnesses and deaths, but annual epidemics caused by influenza A and B viruses have arguably taken a greater toll. Nevertheless, measuring the influenza virus-related burden of disease poses challenges. While influenza A and B viruses cause acute febrile illness, much of the resulting morbidity and mortality is due not to primary influenza illness but to complications and secondary infections not unique to influenza, although most patients no longer have detectable influenza virus when complications develop. Thus, while influenza may lead to exacerbations of many conditions (e.g., chronic obstructive pulmonary disease (COPD), congestive heart failure, and asthma); increased risk of myocardial infarction, cerebrovascular accident, and death due to diverse causes; and increased fetal loss in pregnant women, determining what proportion of such outcomes are attributable to influenza virus is complex, particularly in infants and frail older adults While influenza virus may increase the likelihood of pneumonia and bacteremia caused by S. pneumoniae, H. influenzae b, and S. aureus (including methicillin-resistant S. aureus), these bacterial infections also occur in the absence of influenza virus infection Other challenges to studying the burden of influenzaassociated illness and death include lack of access to sensitive and specific diagnostic tests, difficulty obtaining specimens for testing, the unpredictability of influenza epidemics (and pandemics), and the cost and complexity of assembling and following large cohorts.