2.1 Influenza is an acute infection of the respiratory tract caused by the influenza A and B viruses. The symptoms of influenza are fever accompanied by respiratory symptoms such as sneezing, coughing, runny nose and sore throat and systemic symptoms such as malaise, myalgia, chills and headaches. Gastrointestinal symptoms such as nausea, vomiting and diarrhoea are also common. Influenza infection is usually self-limiting and lasts for 3–4 days, with some symptoms persisting for 1–2 weeks. The severity of the illness can vary from asymptomatic infection to life-threatening complications. The most common complications are secondary bacterial infections such as otitis media, pneumonia and bronchitis.
2.2 Influenza occurs in a seasonal pattern with epidemics in the winter months, typically between December and March. The illness is highly contagious and is spread from person to person by droplets of respiratory secretions produced by sneezing and coughing. Influenza is commonly transmitted through household contacts, with the highest attack rates in children. People who live in residential accommodation and those who work in healthcare settings are at a higher risk of infection. The influenza attack rate is the probability that a person develops influenza over the influenza season. It is expressed as the proportion of people exposed to risk who develop the disease during the period under consideration. The influenza attack rate depends on the circulating level of influenza. It is estimated that yearly influenza epidemics in the UK cause between 12,000 and 13,800 deaths.
2.3 Influenza-like illness, which can be caused by a variety of infectious agents, is a clinical diagnosis made on the basis of symptoms. The causative agent for an influenza-like illness cannot be determined clinically and diagnosis requires laboratory testing. Influenza activity is monitored through surveillance schemes, which record the number of new GP consultations for influenza-like illness per week per 100,000 population. In England, normal seasonal activity is currently defined as 30–200 consultations, with greater than 200 defined as an epidemic. In Wales, the corresponding figures are 25–100, and greater than 400. In addition, there are virological monitoring schemes based on the isolation of the virus from clinical specimens. 'Normal seasonal activity', as measured by these surveillance schemes, corresponds to the term 'circulating' in 'Guidance on the use of oseltamivir and amantadine for the prophylaxis of influenza' (NICE technology appraisal guidance 67). Accurate monitoring of influenza activity requires analysis of clinical, virological and epidemiological information.
2.4 The management of influenza is supportive and consists of relieving symptoms while awaiting recovery. For people in at-risk groups who can start therapy within 48 hours of the onset of an influenza-like illness, treatment with the antiviral drugs oseltamivir or zanamivir is recommended in line with 'Guidance on the use of zanamivir, oseltamivir and amantadine for the treatment of influenza' (NICE technology appraisal guidance 58 [Replaced by NICE technology appraisal guidance 168). Complications require specific management, and antibiotics are used for secondary bacterial infections.
2.5 Vaccination has been established as the first-line intervention to prevent influenza and its complications. In the UK, the Department of Health currently recommends that people who are at risk of influenza infection or complications are vaccinated at the beginning of each winter. Such people are those with chronic respiratory, cardiovascular, renal, liver or neurological disease, people with diabetes, people who are immunosuppressed, people aged 65 and older, people who work or live in residential care facilities, carers of at-risk people, healthcare and other essential workers and poultry workers.
2.6 Antiviral drugs are also used for the prevention of influenza. They may be given to people who have been in contact with a person with influenza-like illness (post-exposure prophylaxis) and may be given in the absence of known contact when it is known that influenza is circulating in the community (seasonal prophylaxis). If seasonal prophylaxis is given, it is carried out for longer periods to cover the duration of the influenza season. Seasonal prophylaxis may be considered in exceptional situations such as an antigenic mismatch between circulating strains of the influenza virus and that used for vaccination which would mean that at-risk people are not effectively protected by vaccination. Prophylaxis may also be used to control outbreaks of influenza within a residential community.