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Epidemiology of Infectious Diseases: Influenza

Causal agent
Influenza is an acute viral disease of the respiratory tract, caused by 3 types of influenza virus: A, B and C.

Influenza A and B are responsible for most clinical illness.

Influenza A is commonly associated with widespread epidemics, type B is infrequently associated with regional or widespread epidemics and type C is commonly associated with sporadic cases1,2.

Common clinical features

  • Symptoms range from asymptomatic infection (20% of infections) to the sudden onset of fever, chills, headache, myalgia, anorexia, muscle aches, sore throat and cough. Up to 30% of infected individuals have upper respiratory symptoms but no fever and up to 25% of children may also have nausea, vomiting or diarrhoea if infected by influenza B or A (H1N1)2,3.
  • In healthy individuals influenza is usually a self-limiting illness lasting up to 7 days.
  • Severe illness and death occur primarily among the elderly and those with underlying chronic cardiac, pulmonary, renal or metabolic disease anaemia or immunosuppresion1. In these groups, the infection may lead to severe complications including bacterial pneumonia and death.


  • In the northern hemisphere influenza occurs during the winter months (December - March).
  • Major genetic changes in the influenza A virus (antigenic shift) occurs at irregular intervals, resulting in the emergence of  new sub-types, that may lead to widespread epidemics or a pandemic in populations who have little or no immunity2.
  • Pandemics have occurred 3 times in the last century.  In 1918-19 'Spanish Flu' which affected large parts of the worlds population is estimated to have caused 20-40 million deaths (a large proportion of which were among healthy young adults).  More recently, two other influenza A pandemics occurred in 1957 (Asian  Influenza) and 1968 (Hong Kong Influenza) which caused significant morbidity and mortality worldwide.
  • Due to frequent minor genetic changes (antigenic drift) in influenza viruses, vaccines are adjusted annually to include the most recent circulating influenza A(H3N2), A(H1N1) and influenza B viruses.
  • The composition of influenza virus vaccines for use each year are determined by the WHO Influenza Surveillance Network. The network, a partnership of 4 WHO Collaborating Centres and 112 National Influenza Centres in 83 countries, is responsible for monitoring the influenza viruses circulating in humans and rapidly identifying new strains.
  • Based on information collected by the network the WHO recommends each year a vaccine to target the 3 most virulent strains in circulation.
  • In the UK influenza activity is monitored through weekly reports of new consultations for 'influenza-like illness' from sentinel GP practices (Royal College of General Practitioners), together with virological surveillance2.
  • In the UK between 3,000 and 30,000 excess winter deaths per year are attributed to influenza3.

Humans are the primary reservoir for human infection.

Birds and mammals such as swine are likely sources of new human subtypes.

Mode of transmission

  • Airborne via droplet infection from coughing and sneezing.
  • Transmission may also occur through fine aerosols and by hand to mucous membrane contact2.

Incubation period
Commonly 1-3 days.

Period of Communicability
From 1 day before and 3-5 days from the onset of symptoms and from 3 days prior and up to 9 days after the onset of symptoms in young children.

Prevention and control
Basic general hygiene (handwashing) to reduce risk of transmission.

Routine immunisation of at-risk individuals is currently recommended in the UK.

The current UK (2005) recommendations are2.

  • All those aged 65 and over.
  • All those aged six months and over in the following risk groups;

Chronic respiratory disease, including asthma.
Chronic heart disease.
Chronic renal failure.
Chronic liver disease.

  • Health and social care staff directly involved in patient care.
  • Those living in long-stay residential care homes or other long stay care facilities.
  • Those who are the main carer for an elderly or disabled persons.  


  1. Heymann D L, editor, Control of Communicable Disease Manual. 18th ed. American Public Health Association; 2004.
  2. Salisbury DM, Begg NT.  Immunisation against infectious disease (The green Book). London: HMSO, 1996. 2005 Update available at:
  3. Hawker J, Begg N, Blair I, Reintjes R, Weinberg J. Communicable Disease Control Handbook, Blackwell, 2005.

Further Resources

NHS, National Institute for Clinical Excellence. Guidance on the use of zanamivir, oseltamivir and amantadine for the treatment of influenza. Technology Appraisal Guidance - No. 58 (2003). Available online at:

© CM Kirwan 2006