Influenza is a common infectious disease worldwide and affects persons of all ages. For example, on average, between 5% and 20% of the population in the United States contracts influenza each year (CDC, 2009b). Most people with the illness recover quickly, but elderly people and those with chronic medical conditions are at higher risk for complications and even death. Between 1979 and 2001, on average, influenza accounted for more than 200 000 hospitalisations and 36 000 deaths per year in the United States (CDC, 2009b). The impact of influenza on the employed population is substantial, even though most influenza morbidity and mortality occurs among the elderly and those with chronic conditions (Keech et al., 1998). In Europe, influenza accounts for around 10% of sickness absence from work, while the cost of lost productivity in France and Germany has been estimated to be in the range of USD 9.3 billion to 14.1 billion per year (Szucs, 2004).