For most people, seasonal influenza causes 2-7 days of mild illness after which they recover without medical treatment.  Seasonal influenza causes severe disease in some people and this group is estimated to be up to 3-5 million people worldwide. Consequently, 250 000- 500 000 deaths yearly are attributable to it.

Influenza is caused by a virus that attacks mainly the upper respiratory tract – the nose, throat and bronchi and, rarely, the lungs.  Seasonal influenza usually affects about 5-15% of the population.

Vaccination of certain groups at high risk for complications is considered the most effective way of preventing severe disease and death. Influenza viruses are constantly changing through a process of antigenic drift, requiring frequent (often yearly) adjustment of the composition of the influenza vaccine to be compatible with circulating virus strains. For this reason, seasonal influenza vaccination needs to be given every year.

Apart from changes in circulating virus strains, the relative dominance of the currently circulating influenza viruses A(H3N2), A(H1N1) and B, as well as the age groups they affect, can change each season. For these reasons, seasonal influenza epidemics can differ in severity and no two influenza seasons are the same. People are recommended for vaccination based on their risk of complications or close contact with people at higher risk of complications.

Risk groups include:

• residents of institutions for elderly people and the disabled;
• people of any age with certain chronic health conditions (such as chronic heart or lung disease, metabolic or renal disease or immunodeficiencies);
• elderly people above a nationally defined age limit; and
• very young children.