Influenza. Cost of illness and considerations in the economic evaluation of new and emerging therapies.

Influenza infection has been a burden to humans for thousands of years. Despite the fact that epidemics could be predicted with regularity, the lack of available prevention or treatment measures left humankind vulnerable to the harmful effects of this ubiquitous virus. While the pandemics of 1918 and 1957 are recent examples of the devastation that influenza may inflict, even in a typical year influenza infection and related complications cause significant morbidity and mortality. The development of an influenza vaccine during the 1940s marked a major turning point in the management of this disease. Vaccination of the elderly and other high risk patients has been shown to reduce morbidity and mortality and to be a worthwhile investment from an economic perspective. Despite these benefits, vaccine use in this group remains suboptimal. The role of annual vaccination for individuals at lower risk for influenza-related complications remains controversial. While prevention by vaccination is relatively straightforward, the treatment of symptomatic influenza-like illness with medication is more complicated. Differentiating symptoms caused by the influenza viruses from those caused by other common viruses is difficult. Currently available tests to document influenza as the cause of illness are either too expensive, too inaccurate or too time consuming to impact treatment. Symptom-based diagnosis remains the most commonly used strategy in clinical practice. The approval of the neuraminidase inhibitors (NIs)--zanamivir and oseltamivir--remind healthcare providers of the difficulties in diagnosing and treating influenza. NIs have been shown to reduce the duration of symptoms of individuals infected with influenza when prescribed within the first 2 days of symptoms. Whether these innovative agents are cost effective, however, requires a more detailed understanding of the benefits that these agents may offer above and beyond existing therapies. In this review, we examine the burden of influenza infection, diagnostic challenges and the clinical and economic impact of available interventions. Clinical controversies and potential areas for further investigation are also explored.