Influenza usually occurs during annual epidemics and occasional pandemics, the most recent pandemic being in 2009. Rates of infection are highest among children, with rates of serious illness and death highest in individuals over 65 years of age, children under 2 years of age, and persons of any age who have medical conditions that predispose to increased risk of complications from influenza. More than 90% of influenza-related deaths occur in patients in the older age group. Underlying medical conditions that increase the risk of hospitalisation with seasonal influenza include diabetes and cardiovascular, neurological and chronic respiratory diseases, such as asthma.
There are varying estimates of the number of people infected by influenza every year, the resulting burden of ill health and premature death, and the degree to which these burdens can be reduced, but it is agreed that influenza is a significant threat to public health.
The attack rates during seasonal influenza epidemics can vary considerably from year to year, but usually some 5–20% of the population is affected. The impact of influenza in different age groups varies considerably between epidemics. Usually, winter epidemics affect each country for 1–2 months and, across Europe, epidemics last for about 4 months. The epidemiology and clinical features of influenza can differ during pandemics, depending on the characteristics of the virus and on the level of immunity to a virus that by definition is different from those circulating in previous influenza seasons. This was the case during the 2009 influenza A (H1N1) pandemic, where the highest incidence of infection and disease was in younger individuals (i.e. those less than 65 years of age).
Figure 2 shows the case-load during the H1N1 pandemic in Europe.
Post-influenzal bacterial pneumonia is a major cause of morbidity and mortality associated with both seasonal and pandemic influenza.
Causes and pathogenesis
Influenza in humans is caused by three major families of RNA viruses: influenza A, B and C. They are usually classified according to differences in the antigenic properties of their external coat. Influenza A viruses, clinically the most important, are further divided into subtypes based on two proteins on the external coat, the haemagglutinin (HA) (H1–H16) and neuraminidase (NA) (N1–N9) proteins. Type B viruses cause somewhat less severe illness, and type C viruses do not cause significant human disease, so only type A and B viruses are of concern. Another important challenge is the emergence of influenza virus strains resistant to antivirals.
Clinical manifestations and consequences
Influenza viruses can cause disease among persons in any age group but the frequency of infection is highest in children. Rates of serious illness and death are highest among individuals over 65 years of age, children less than 2 years of age and persons of any age who have medical conditions that increase the risk of complications from influenza. Uncomplicated influenza is characterised by the abrupt onset of constitutional and respiratory signs and symptoms (e.g. fever, myalgia (muscle pain), headache, malaise, nonproductive cough, sore throat and rhinitis). Among children, otitis media, nausea and vomiting are also commonly reported with influenza infection. Uncomplicated influenza typically resolves after 3–7 days in most people. However, influenza virus can cause primary influenza viral pneumonia, exacerbate underlying medical conditions (e.g. pulmonary or cardiac disease), lead to secondary bacterial pneumonia, sinusitis and otitis media, or contribute to co-infections with other viral or bacterial pathogens.
Two classes of antivirals, the adamantanes (amantadine and rimantadine) and the neuraminidase inhibitors (laninamivir, oseltamivir, peramivir, and zanamivir), are currently approved for the prevention and treatment of influenza; several other classes of antivirals and immune modulators are also under investigation.