Influenza – estimating burden of disease

During the winter months, seasonal influenza can infect up to 20% of the population, depending on which viruses are circulating, and can cause substantial mortality. A recent study found that worldwide up to 650 000 people die of respiratory diseases linked to seasonal influenza each year, and up to 72 000 of these deaths occur in the WHO European Region.

While it is generally recognized that influenza has a significant economic impact in the form of health-care costs and lost working hours, it is often challenging for countries to estimate the full economic impact of morbidity and mortality from influenza. Furthermore, the burden of influenza will vary year to year, depending upon which viruses are circulating and which people are affected.

Some of the challenges of estimating burden of disease include:

• the difficulty of distinguishing influenza from other respiratory illnesses without good laboratory testing;
• the fact that much of the morbidity and mortality resulting from influenza is due to complications and infections not unique to influenza, many of which may not be captured in seasonal influenza surveillance data; and
• incomplete and low-quality surveillance data from which estimates are made.

Consequently, there is a need for reliable national disease burden estimates for influenza to:

• provide a better understanding of the impact of influenza, especially in vulnerable communities or subpopulations such as young children, people 65 years and older, pregnant women and people with underlying illness;
• make informed, evidence-based decisions when allocating scarce resources (for example, for the acquisition of influenza vaccines) and when planning intervention strategies to limit the spread of influenza and minimize its associated costs; and
• evaluate the impact of interventions (for example, illness-related expenditure and hidden costs averted by influenza vaccination).

WHO/Europe supports ongoing efforts to reduce influenza-related morbidity and mortality by strengthening surveillance of mild and severe influenza, and by assisting countries to use these surveillance data, along with vital statistics such as mortality data, to estimate the burden of seasonal influenza.