In the elderly and at-risk groups, the most common complication of influenza infection is secondary bacterial pneumonia, caused by respiratory pathogens such as Streptococcus pneumoniae, Hemophilus influenzae and Staphylococcus aureus. Other complications include myocarditis and the worsening of chronic medical conditions, such as asthma or diabetes.

The clinical features of influenza infection are similar in children, although their fever may be higher and febrile convulsions can occur. In addition, children have a higher incidence of vomiting and abdominal pain as well as otitis media complications, croup and myositis.

While the majority of people will recover from influenza infection within one to two weeks without requiring any medical treatment, for certain members of the population the disease may present a serious risk. Such individuals include the very young, the elderly and people suffering from medical conditions such as lung diseases, diabetes, cancer, kidney or heart problems. In these people, the infection may lead to severe complications of underlying diseases, pneumonia and death.

The WHO estimates that in annual influenza epidemics 5-15% of the population is affected with upper respiratory tract infections. Hospitalization and deaths mainly occur in high-risk groups (elderly, chronically ill). Although difficult to assess, these annual epidemics are thought to result in between three and five million cases of severe illness and approximately 250 000 and 500 000 deaths every year around the world. Over 90% of the deaths currently associated with influenza in industrialized countries occur among the elderly over 65 years of age.

In the U.S., the CDC estimates that more than 200,000 people are hospitalized every year on average following complications arising from influenza infection, with around 36,000 excess mortalities being recorded.

• WHO Influenza Key Facts
• CDC Influenza Key Facts