After the second world war, vaccination became the main strategy for preventing and controlling seasonal and pandemic influenza worldwide. European Respiratory Society (ERS) and European Society of Clinical Microbiology and Infectious Diseases (ESCMID) guidelines define influenza as an acute illness, usually with fever, with one or more of the following: headache, myalgia (muscle pain), cough or sore throat. Influenza is caused  by influenza viruses. While most illnesses are brief and without consequence, regular seasonal epidemics of influenza include significant rates of severe illness and death, particularly among elderly people and those with underlying chronic medical conditions. Type A virus causes the most severe disease and is associated with epidemics and pandemics. Spontaneous mutations in the viral surface proteins, haemagglutinin and neuraminidase, are responsible for so-called ‘antigenic drift’. If this results in changes in the viral amino acid structure, pre-existing antibodies might be unable to bind to viral particles to a sufficient extent to prevent disease. This phenomenon is responsible for the annual influenza waves observed worldwide.

Avian influenza infections are much more severe than the common seasonal influenza, and are associated with severe illness and a mortality rate exceeding 50%. Occasionally, a new strain develops to which many humans have little or no immunity and a worldwide pandemic occurs, as was seen in 2009. This pandemic, caused by the influenza A (H1N1) virus, spread in two waves, a modest spring/summer wave and a more sustained wave in the autumn and early winter with moderate intensity. Almost all influenza cases in 2009 were caused by the pandemic virus. Surveillance of hospitalised acute respiratory cases was implemented in various forms by 10 European Union (EU) countries during the pandemic, with 9469 laboratory-confirmed cases reported and 569 deaths. Severe disease was more frequent, and the death rate was higher in individuals under 65 years of age and in those with underlying disease, even though in 25% of the severe cases there were no underlying conditions.

Seasonal influenza vaccine has proved effective in preventing laboratory-confirmed influenza among healthy adults (16–65 years of age) and children (6 years of age or older). The evidence of vaccine effectiveness is much more limited in relation to the prevention of complications such as pneumonia, hospitalisation and influenza-specific and overall mortality. However, vaccinating children might have a protective effect for nonrecipients of the vaccine of all ages living in the same community as it prevents transmission. Scientific evidence suggests there would be advantages to vaccinating older people and those with chronic disease.

The ERS/ESCMID guidelines recommend that influenza vaccine should be given yearly to people at increased risk of complications due to influenza. Vaccination should be given to immunocompetent adults belonging to one or more of the following categories: over 65 years of age; resident in an institution (such as a nursing home); chronic cardiac disease; chronic lung disease; diabetes mellitus; chronic renal disease; haemoglobinopathies; and women who will be in the second or third trimester of pregnancy during the influenza season. In addition, the guidelines suggest yearly vaccination for healthcare personnel, especially in settings where elderly people or other high-risk groups are treated. Figure 1 shows the influenza vaccination rates in Europe in people over 65 years of age in the 2008–2009 influenza seasons.

See the entire Immunisation against respiratory diseases Chapter