Deaths attributable to influenza difficult to estimate

The numbers of deaths that are attributable to influenza are difficult to estimate directly for several reasons (Thompson et al., 2003):

• Influenza is typically not confirmed virologically as an influenza virus infection or specified on hospital discharge forms or death certificates.
• Many influenza-associated deaths occur from secondary complications such as congestive heart failure, COPD, pneumonia, and bacterial superinfections, when influenza viruses are no longer detectable (Bisno et al., 1971; Douglas, 1976).
• Respiratory syncytial virus (RSV) epidemics often overlap with influenza epidemics. It is likely that some deaths previously attributed to influenza are actually associated with RSV infection (Nicholson, 1996; Zambon et al., 2001; Fleming & Cross, 1993).

Slight increase in deaths from flu over the last decades

Some countries, including Germany and the United States, have reported an increase in absolute mortality numbers by influenza since the early eighties. This is probably, or at least in part, due to aging of their populations (Thompson et al., 2003 and Zucs et al., 2005) as mortality rates for influenza are higher in the elderly.

Vaccination is associated with reduced mortality

Influenza is an infectious disease that can induce inflammatory responses (Naghavi et al., 2003; Julkunen et al., 2000). Inflammation is highly associated with increasing mortality from cardiovascular diseases, diabetes mellitus, and stroke (Nathan, 2002; Libby, 2002; Engstrom et al., 2003).

Several studies have found that influenza vaccination is strongly associated with reducing lung disease-specific mortality, including deaths by pneumonia and COPD. In addition, vaccination is associated with a reduction in all-cause mortality and in other major causes of death, such as stroke, diabetes mellitus, and renal disease.

In a study by Wang et al. it was suggested that the protective effect of vaccination might extend beyond the influenza season to the rest of the year. Therefore, the influenza vaccination not only reduces the mortality risk for influenza-related pneumonia and COPD, which occur mainly during influenza season, but also long-term complications such as stroke, cardiovascular diseases, or other major causes of death. These long term complications might occur during or after the influenza season, or in later months, due to secondary complications from influenza (Wang et al., 2007).

Highest pandemic excess mortality rates during the 1918 Spanish flu

Pandemics have occurred three times in the previous century. The 1918 `Spanish flu' A(H1N1) pandemic was particularly severe. Medical historians have generated estimates of mortality of about 50 million and ranging from 20 million to 100 million. These estimates are based on reviews of various historical documents, including national commissions, eye-witness accounts, and local government reports (Murray et al., 2006). The more recent pandemics, A(H2N2) `Asian flu' in 1957 and A(H3N2) `Hong Kong flu' in 1968, were associated with moderately increased mortality rates (Simonsen, 1999).