Influenza is an acute viral infection of the respiratory tract that affects the upper airways and lungs. There are three types of influenza virus: A, B and C. Influenza A and influenza B are responsible for most clinical illness. Influenza is highly infectious with a usual incubation period of one to three days.

For otherwise healthy individuals, influenza is an unpleasant but usually self-limiting disease with recovery usually within two to seven days. Influenza may be complicated by, and may present as, bronchitis, secondary bacterial pneumonia or, in children, otitis media.

Serious illness and mortality from most influenza viruses are highest among:

• neonates
• older people
• those with underlying disease, particularly chronic respiratory or cardiac disease
• those who are immunosuppressed

Early symptoms of influenza include the sudden onset of fever, chills, headache, myalgia and extreme fatigue. Influenza can also be complicated by meningitis, encephalitis or meningoencephalitis. 

Use the ‘Symptoms Checker’ below to rollover and reveal influenza, symptoms and signs.

The information outlined above reflect current NHS Choices recommendations 

How does disease transmission occur?

• By aerosol, droplets or direct contact with respiratory secretions of someone with the infection
• Influenza spreads rapidly, especially in closed communities

Prevent spread of infection

It’s important to try and stop influenza viruses spreading to protect others from the problems it causes. So individuals should practise good hygiene such as sneezing and coughing into tissues, washing hands and hard surfaces frequently and keeping away from crowded places to avoid passing the infection on.

2009/10 Pandemic

Influenza A (H1N1) virus caused the 2009/10 pandemic.
Characteristics of the recent pandemic were:

• Higher rates in children and young adults
• Lower rates in adults aged 60 years and older
• For most, the disease was mild
• Symptoms were similar to those of seasonal influenza, although gastrointestinal symptoms (vomiting and diarrhoea) were more common
• Most of the serious complications occurred in:
  • people with underlying health conditions
  • those with chronic neurological disease, respiratory disease and immunosuppression
  • pregnant women
  • people who had been healthy

To view the latest Department of Health ‘at risk’ group recommendations for influenza click here.

Influenza immunisation has been recommended in the UK since the late 1960s, with the aim of directly protecting those in clinical risk groups.

GP consultations for influenza-like illness per 100,000 population in England.

For six of the last seven years, the influenza season occurred between weeks 37 and 15 but much of the influenza A (H1N1) activity in 2009/10 was outside this time window.

WHO monitors influenza viruses

Because of the changing nature of influenza viruses, the World Health Organisation (WHO) monitors the epidemiology of influenza viruses throughout the world. Each year it makes recommendations about the strains to be included in vaccines for the forthcoming winter.

Influenza vaccines are prepared using virus strains in line with the WHO recommendations. Annual immunisation with the latest prepared vaccine will provide your ‘at risk’ patients with the best protection.

Manufacture is complex and conducted to a tight schedule, constrained by the length of time available between the WHO recommendations and the opportunity to vaccinate before the influenza season.

Current Influenza vaccine

The 2011/12 seasonal influenza vaccines are trivalent, containing two subtypes of influenza A and one type B virus.

A/California/7/2009 (H1N1)-like virus;
A/Perth/16/2009 (H3N2)-like virus;
B/Brisbane/60/2008-like virus.

Two types of trivalent seasonal influenza vaccine are currently licensed and available for use in the UK:

• ‘Split virion, inactivated’ or ‘disrupted virus’ vaccines containing virus components prepared by treating whole viruses with organic solvents or detergents
• ‘Surface antigen, inactivated’ vaccines containing highly purified haemagglutinin and neuraminidase antigens prepared from disrupted virus particles

Points to remember

• Manufacturers may not be able to respond to unexpected demands for vaccine at short notice
• The vaccines are inactivated, do not contain live organisms and cannot cause the diseases against which they protect
• Not all influenza vaccines are indicated for all age groups

Pfizer Vaccines supply two flu vaccines in the UK, Enzira (Influenza vaccine, split virion, inactivated) and a generic influenza vaccine, split virion, inactivated. Click here to view the product information. The product information is the same for both vaccines.

Due to an increased risk of febrile convulsions, Enzira and the generic influenza vaccine are not authorised for use in children under 5.

Did you know?

• Influenza A is the usual cause of epidemics. Influenza B tends to cause less severe disease
• Three influenza pandemics occurred in the last century (in 1918, 1957 and 1968)
• The first influenza pandemic of this century was in 2009 and caused by influenza A (H1N1)
• After immunisation, antibody levels may take up to 10 to 14 days to reach protective levels
• Flu vaccinations prevent about 70-80% of influenza in healthy adults