Influenza Burden of Illness

Abstract

INFLUENZA EPIDEMICS have an important and well-documented impact on mortality, particularly in at-risk individuals, including elderly persons.^1-4 Recent work⁵ suggests that influenza epidemics are also associated with an elevated number of hospitalizations. On the other hand, their impact on uncomplicated morbidity and their consequences on health care utilization and lost productivity are still relatively unclear, despite the development of influenza surveillance systems in many countries. These systems are based on data collected from physicians who report cases of syndromes meeting a clinical case definition of influenza, sometimes but not always combined with virologic testing; they make it possible to detect epidemics early, to identify the circulating viral strains, and to supply quantitative information, for example, about the number of "excess" physician visits associated with an epidemic.^6,7 For several reasons, however, the results of this surveillance yield an incomplete picture of the true impact of epidemics. The clinical expression of the infection caused by influenza viruses is variable, and some physician visits for influenza infection will not be counted (and no sample will be taken) because the patient's clinical presentation does not meet the case definition. Carrat et al⁸ previously showed that nearly half of the patients visiting a physician for influenza had an attenuated form of the disease. Moreover, some individuals with influenza do not visit physicians; this fraction may represent 30% to 65% of patients whose clinical picture suggests influenza.^9,10 These individuals cannot be captured by a medically based surveillance system. At the other end of the spectrum, some patients may visit a physician several times for the same influenza episode,¹¹ and these new visits are rarely counted. Failure of the surveillance systems to take into account these categories of patients leads to a potentially important underestimation of the epidemic's impact on uncomplicated morbidity. This knowledge is nonetheless essential, especially for an accurate evalua tion of the clinical and economic benefits of preventive and treatment strategies against influenza.^12,13