Influenza is a viral disease of the airways

Influenza is an acute viral disease of the airways that occurs especially during the cold seasons. Widespread outbreaks of influenza are caused by influenza viruses type A or B. Type A influenza viruses are further subtyped by their surface antigens hemagglutinin (H1-H16) and neuraminidase (N1-N9), e.g. A(H3N2). The infection spreads through droplets (aerosols) that are generated by coughing or sneezing. The disease often begins with fever and chills, accompanied by headache and sore throat, muscular pain, general bodily discomfort, loss of appetite, and a dry cough. Fever (38–40°C) peaks within 24 h of onset and lasts 1–5 days. Healthy people overcome influenza within 1 to 2 weeks. Influenza is sometimes associated with severe complications such as pneumonia, worsening of chronic medical conditions, acute otitis media and sinus problems in children. Influenza can increase mortality in groups with high risk for complications following influenza, e.g., the elderly population.

Seasonal influenza is very common

During seasonal influenza epidemics 5-15% of the population in the Northern hemisphere is affected. In Europe the European Centre for Disease Prevention and Control (ECDC) publishes data about seasonal influenza activity, intensity and geographical spread in EU-27 countries and Norway (European Influenza Surveillance Network; EISN), and the WHO European regional office for the 52 WHO Europe region (EUROFLU).

Virus epidemic and pandemic

From time to time new influenza A or B virus variants emerge due to genetic changes in the virus that cause alterations in the antigens on the surface of the virus (antigenic drift). The virus can cause almost every year a new epidemic, which can be widespread. There is also a possibility that a completely new subtype of influenza A virus will emerge (antigenic shift). Since it is completely new, people are naturally not immune protected against it at all, and therefore this can lead to a widespread epidemic, a so-called pandemic.

Numbers of deaths attributable to influenza are difficult to estimate

The numbers of deaths attributable to influenza are difficult to estimate directly. It is estimated by ECDC that at least 40,000 people die each year from influenza in the EU. Some countries, including Germany and the United States, reported since the early eighties an increase in absolute mortality by influenza probably (at least in part) due to aging. Vaccination is associated with reduced all-cause mortality and other major cause-specific mortality (e.g. pneumonia, COPD, stroke, diabetes mellitus, renal diseases).

Influenza pandemics have occurred three times in the previous century. The 1918 `Spanish flu' A(H1N1) pandemic was particularly severe with estimated mortality ranging from 20 million to 100 million. The more recent pandemics, A(H2N2) `Asian flu' in 1957 and A(H3N2) `Hong Kong flu' in 1968, were associated with moderately increased mortality.

Some people have increased risk for severe complications due to influenza

Those aged 65 years and older and persons of any age with chronic medical conditions (such as asthma, diabetes, or heart disease) are at higher risk for serious complications of influenza. Elderly people have an increased risk of a permanent reduction in their physical and mental capabilities. In some cases their death is attributed to secondary bacterial pneumonia, but sometimes death occurs without any apparent complication. Approximately 20% of children aged 6 months to 5 years of age that were hospitalized with influenza A virus infections had febrile seizures. Pregnant women are at slightly increased risk of serious pneumonia due to a heightened location of the diaphragm in the course of pregnancy.

In the recent decades, a number of developments have led to a reduction in the number of influenza cases and deaths due to influenza (e.g., improved general health, a decrease in the number of persons per household and vaccination in particular among risk groups). On the other hand some developments, such as the rise of social contacts, accommodation of young children in nurseries and increased (global) mobility have increased the risk for getting infected.

Vaccination is the most effective way to prevent influenza

The most effective way to prevent influenza, complications and death is by vaccination. Vaccination is especially recommended for groups at risk for complications following influenza such as the elderly and people with chronic medical conditions, such as lung and heart disease, and people whose immune systems are weak. There is low coverage of influenza vaccination in the elderly in many European countries. Antiviral drugs, such as amantadine and rimantadine and the newer drugs oseltamivir and zanamivir, reduce the severity of illness and disease duration. However, they are prescribed only in special cases, e.g. patients with influenza at risk for complications. Vaccination remains the best way to prevent influenza.