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seasonal influenza viruses

This Factsheet is also available as a pdf.


A wide variety of influenza (flu) viruses circulate in mammals and birds, of which there are many strains. Seasonal influenza is caused by groups of influenza viruses that can spread between people, with outbreaks typically occurring each winter.



  • Currently there are two subtypes of Influenza types A and two different lineages of influenza type B that cause seasonal influenza outbreaks.
  • Influenza type A subtypes are named according to the two proteins on the virus surface: haemagglutinin (H) and neuraminidase (N) (e.g. the H1N1 strain). These proteins are the main target of protective immunity and are included in influenza vaccines.



  • Seasonal influenza outbreaks occur in winter when conditions are best suited to viral survival and transmission. Low temperatures allow the virus to survive for longer, and combined with low humidity makes transmission easier.
  • New strains of virus evolve rapidly because of the large replication rate, high rate of mutation, and ‘cross-breeding’ between influenza viruses (called viral reassortment).
  • Immunity to previously circulating influenza viruses promotes the emergence of new viruses that are different enough to infect people previously exposed to older viruses.



  • Influenza is usually spread in droplets that are breathed out from an infected person, or projected into the air during coughing or sneezing. The virus is taken up by cells lining the nose, throat and lungs and replicates within these cells. An immune response is triggered by the infection, which causes fever, chills, cough, headache, muscle ache and tiredness. This usually occurs 2-7 days after infection and lasts 2-4 days.
  • Anyone can be infected but young, old, or immunosuppressed people may not have strong enough immune systems to deal with the virus. People who are already unwell (e.g. respiratory problems) or pregnant are more at risk of complications from infection.



  • The constant emergence of antigenically different viruses necessitates annual updates of the composition of seasonal influenza vaccines. This is coordinated by the World Health Organization and influenza experts around the world who monitor the evolution of influenza viruses, and the European Medicines Agency.
  • Two different forms of influenza vaccine are used in the UK: injected or nasal spray. The vaccines may be produced in eggs, so are not recommended for those with egg allergy.
  • The injected vaccine contains an inactivated ‘dead’ form or fragment (HA) of the virus of the virus. This type of vaccine is recommended for adults over the age of 65, pregnant women, healthcare workers and those with ongoing health problems.
  • The influenza vaccine given as nasal spray contains a weakened live virus that can replicate but is unable to cause disease. This vaccine is relatively ineffective in those who have had flu in the past, since it needs to infect locally to induce immunity. It is therefore only currently being offered to children aged 2-4, or people aged 2-17 who are at risk from infection. This type of vaccine may give broader immunity than the inactivated (injectable) vaccine.
  • The injected vaccine is expected to protect around 75% of people from influenza, and to reduce the severity of flu in those who do become infected despite the vaccine.
  • 1-2% of people receiving the influenza vaccine will develop an ache in the injected arm or mild flu-like symptoms and tiredness. These symptoms should subside in 1-2 days and are due to the immune response. It is not possible for the injectable vaccine to actually cause flu.
  • Antibiotics should not be used to treat uncomplicated influenza, as they are only effective against bacteria and not viruses.
  • Antiviral drugs (such as oseltamivir (Tamiflu) and zanamivir (Relenza)) help flu symptoms to resolve more quickly, reducing illness duration by around 1 day. They are not necessary for healthy people, but in people with severe symptoms or at high risk of complications they can be beneficial (especially if used soon after the onset of symptoms).


Prevalence and impact

  • Globally, influenza affects an estimated 5-10% of adults and 20-30% of children each year. Worldwide influenza may result in about 3-5 million cases of severe illness and 250,000-500,000 deaths (from these figures).
  • Each winter in the UK tens of thousands are hospitalised because of flu, and an average of around 8,000 deaths attributed to flu (from data here). Last season, 49% of outbreaks were from care home settings, 39% in hospitals and 9% in schools (from these figures).
  • Seasonal flu in England and Wales accounts for 400,000 GP consultations per year. Every winter, 11,000 elderly respiratory hospital admissions cost the NHS over £22 million.


Antigen: a substance which stimulates an immune response.


Sources / further information

Centers for Disease Control and Prevention seasonal influenza

ESWI economic impact assessment

Flu News Europe

Mortality Attributable to Influenza in England and Wales Prior to, during and after the 2009 Pandemic, Green et al.

NHS flu information

NHS flu treatment

NHS flu vaccination

PHE report on 2013-14 seasonal flu

Weekly National Influenza reports by Health Protection Scotland

Weekly National Influenza reports by Public Health England

World Health Organization seasonal influenza fact sheet



This is a Factsheet issued by the Science Media Centre to provide background information on science topics relevant to breaking news stories. This is not intended as the ‘last word’ on a subject, but rather a summary of the basics and a pointer towards sources of more detailed information. These can be read as supplements to our Roundups and/or briefings.



Updated 01/12/2014

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