Seasonal influenza

Influenza is a highly infectious, viral infection of the respiratory tract; outbreaks peak during the winter months of the northern and southern hemispheres, and occur year around in the tropics Seasonal influenza


Key Messages

Seasonal influenza is a viral infection, spread from person to person through respiratory droplets.
Seasonal influenza occurs throughout the world; outbreaks peakduring the winter months of the northern and southern hemispheres, and occur year around in the tropics.
Influenza is usually self-limiting with recovery within one to two weeks; severe illness and death can occur, particularly in those with pre-existing medical conditions.
The three main ways to prevent seasonal influenza are good cough and hand hygiene, vaccination, and antiviral medication.
Annual vaccination is recommended for some individuals prior to the start of the influenza season in order to provide protection to those who are at higher risk of severe illness. Antiviral medications are used for those in ‘at risk’ groups as defined by Public Health England [1].


Influenza is a viral infection of the respiratory tract; symptoms appear rapidly but last a relatively short period of time. In healthy individuals influenza is usually self-limiting (i.e. it resolves without treatment), with recovery within one to two weeks [1, 2].

There are three types of seasonal influenza viruses, types A, B, and C [1, 2]. Influenza A and B viruses cause outbreaks and epidemics. Only influenza type A viruses are known to have caused pandemics (i.e. worldwide spread of a new disease) [2, 3]. Influenza type C virus usually causes mild infections and is detected much less frequently [2].

In April 2009, an influenza A/H1N1 virus of swine origin was the cause of a global pandemic [4]. In August 2010, the World Health Organization (WHO) declared that pandemic influenza A/H1N1 (2009) had entered the post pandemic phase [5]. However, the A/H1N1 (2009) virus is expected to take on the behaviour of a seasonal influenza virus and continue to circulate for many years [5]. The relevant strains influenza viruses to be included in seasonal vaccines are determined each year by the World Health Organization (WHO) [6].

Risk areas

Seasonal influenza occurs throughout the world. In temperate regions of the northern hemisphere most influenza activity is from November to March (between December and March in the United Kingdom). In the southern hemisphere most influenza activity occurs between April and September. In the tropics, influenza viruses can circulate throughout the year.

Up to date worldwide information on seasonal influenza activity is available from the World Health Organization (WHO). Data on seasonal influenza activity in Europe is also available from the European Centre for Disease Prevention and Control.

Risk for travellers

Influenza has been described as the most frequent vaccine preventable infection among travellers to tropical and subtropical countries [7-9].

The risk of exposure to influenza during travel depends on time of year, type of travel, destination and duration. However, travellers can be at risk during the summer months at their destination, particularly if travelling in large groups that include tourists from other regions of the world where influenza viruses are currently circulating, such as on cruise ships [10]. Crowded conditions accelerate the spread of infection (e.g. Hajj or Umra pilgrimages or cruise travel).

There is limited data on foreign travel-related cases of influenza in the UK, as travel history is not routinely collected for seasonal influenza cases. One indicator of a travel-related case would be the isolation of an influenza virus not known to be circulating in the UK or the isolation of a novel virus subtype.


Influenza virus spreads easily from person to person via respiratory droplets when coughing and sneezing. Crowded, enclosed environments facilitate transmission [1]. Another potential route of infection is from touching surfaces or objects contaminated with respiratory droplets and then touching the mucous membranes on the nose or mouth [1, 2, 7].

Signs and symptoms

Classic symptoms of influenza are the sudden onset of fever, chills, headache, cough (usually dry), extreme fatigue, sore throat, runny nose and muscle and joint pain [1, 2]. Influenza can affect all age groups, and the burden of disease on each age group can vary from season to season, depending on the strains circulating.

Although infection is usually self-limiting, it can lead to complications, secondary infections and exacerbation of underlying medical conditions, which can be fatal. The elderly, the very young and those with serious medical issues (e.g. chronic (long-term) heart conditions, chronic respiratory conditions and immunosuppression), are particularly at risk. Very rare complications of influenza can be encephalitis (inflammation of the brain) and meningitis (infection of the protective membranes that surround the brain and spinal cord) [1].

Pandemic influenza A/H1N1 (2009) generally caused a mild disease in children and young adults, however, severe cases and deaths did occur in these age groups. In the UK, most deaths were in those younger than 65 years old, the majority of whom suffered from an underlying illness, although a number of deaths in previously healthy individuals were documented [11]. Pregnant women were also at higher risk of severe illness [12].

Diagnosis and treatment

During the influenza season, diagnosis is usually made by consideration of clinical signs and symptoms; specific testing for influenza virus (laboratory diagnosis) is usually only indicated in cases of complicated influenza [13], or for surveillance purposes.

In the UK there are licensed antiviral drugs that can be taken to prevent or treat influenza, Public Health England (PHE) guidelines are available [13].

Preventing seasonal influenza

Vaccination against influenza is the most effective way of preventing the illness [1]. See ‘Vaccine information’ section for details on who should be offered this vaccine.

Travellers are advised to take the following precautions to reduce their risk of exposure to respiratory infections and prevent spreading them:

  • Avoid close contact with symptomatic individuals
  • Avoid crowded conditions where possible
  • Practise frequent hand washing
  • Practise ‘cough hygiene’: sneezing or coughing into a tissue and promptly discarding it safely, and frequent hand washing
  • Avoid travel if unwell with influenza-like symptoms

 In the UK there are licensed antiviral drugs that can be taken to prevent influenza for those in “at risk” groups as per PHEs guidance [1, 13].

Vaccine information

In the UK, influenza vaccines are prepared using virus strains recommended annually by WHO. The vaccine formulation is reviewed and changed as necessary to provide protection against strains of influenza viruses that are predicted to circulate in a given season. Information on epidemiological trends and circulating influenza viruses are gathered by WHO, to ensure the closest possible match between prevalent influenza viruses and influenza vaccines [6].

In the UK, influenza vaccines are prepared in advance of the northern hemisphere winter season. The vaccines are either trivalent (containing two influenza A subtypes and one influenza B subtype) or quadrivalent (containing two influenza A subtypes, and two influenza B subtypes).

Most vaccines are grown in embryonated hen’s eggs. No currently administered vaccine offers protection against highly pathogenic avian influenza A H5N1. All but one influenza vaccine currently used in the UK are inactivated and do not contain live viruses. One vaccine, the intranasal administered vaccine (a nasal spray squirted up each nostril), contains attenuated (weakened), cold adapted viruses, which cannot replicate at body temperature. The vaccines therefore cannot cause influenza.

Following vaccination, protection is thought to last for approximately one year, although this may be less for the elderly. After vaccination, antibody levels take 10 to 14 days to reach protective levels [1].

Further information on the UK’s influenza programme, including information about vaccine virus strains, is available from Public Health England.

Indications for use of vaccine

The aim of the UK’s influenza programme is to protect those most vulnerable to serious illness or death if they develop influenza. The UK has also recently started the introduction of the offer of influenza vaccine for healthy children. Influenza vaccine becomes available annually in the UK in September and is offered to [1, 14]:

  • All those aged 65 years and older
  • Children aged two to eight years of age

All those aged six months and older in the risk groups listed below:

  • Asplenia (absent spleen) or splenic dysfunction (spleen not fully functioning)
  • Chronic (long-term) respiratory disease
  • Chronic heart disease
  • Chronic renal (kidney) disease
  • Chronic liver disease
  • Chronic neurological (nervous system) disease
  • Diabetes
  • Immunosuppression (weakened immune system)
  • Morbidly obese (defined as having a BMI of 40 and above)
  • Pregnant women

In addition, vaccination is recommended for:

  • Those living in long stay care facilities
  • Health care and social care workers with direct patient or service user contact
  • Carers of disabled or vulnerable individuals
  • Household contacts of people with weakened immune systems

The list above is not exhaustive. Public Health England states that influenza vaccine can be offered to others based on the clinical judgement of a health professional [1]. 

In the UK, vaccination is not routinely recommended for travellers. Health professionals should carefully assess the risk of influenza for healthy travellers to tropical and sub-tropical regions during the influenza season at the destination. A decision to vaccinate healthy adults could be made but the vaccine cannot be given as an NHS service for this category of traveller. In the UK, northern hemisphere influenza vaccines for the winter season usually become available in October. Most of the stock is used over the winter and supplies may not be available during the spring and summer months. Currently, the southern hemisphere vaccine (which may contain different strains of influenza viruses as recommended by WHO) is not available in the UK.

See Immunisation against infectious disease (The 'Green book') for further information on indications for influenza vaccine.

Vaccine schedules

The updated influenza chapter of Immunisation against infectious disease (‘Green Book’) should be consulted for current vaccine schedules and dosages.

Detailed vaccine information can also be found in the manufacturer’s Summary of Product Characteristics (SPC) on the electronic Medicines Compendium (eMC).


Very few individuals are unable to receive any influenza vaccine. As with all vaccines, anyone with a moderate to severe acute febrile illness should delay vaccination until they have recovered. The vaccine should not be given to anyone with a confirmed anaphylactic reaction to a previous dose of the vaccine, or to any component of the vaccine [1].

Influenza vaccines with egg albumin content greater than 0.12 μg/ml, or where the content is not stated, should not be given to egg allergic individuals. Some influenza vaccines are either ‘egg free’ or have very low ovalbumin content. The ovalbumin content of available influenza vaccines can be found in the ‘Green Book’ [1]. Children and adults with either confirmed anaphylaxis to egg or with egg allergy and severe uncontrolled asthma should be referred to a hospital specialist; vaccination, if considered appropriate, should be done in a hospital setting [1]. All other egg allergic adults can be vaccinated in primary care using egg-free influenza vaccine or influenza vaccine with an ovalbumin content less than 0.12 μg/ml [1, 15].

The live attenuated vaccine should not be given to immunosuppressed individuals, those with severe asthma, or active wheezing at the time of vaccination. Pregnant women should be offered an inactivated vaccine.

Practitioners should refer to the ‘Green Book’ for further guidance and seek specialist advice if appropriate.

Adverse events

Transient reactions such as soreness, swelling or redness at the site of injection can occur. Fever, malaise and other systemic symptoms are also reported [1].

Convulsions, neuralgia (nerve pain), paraesthesia (abnormal sensation such as tingling or prickling) and transient thrombocytopenia (temporary low platelet count in the blood) have been rarely reported [1]. A recent study in the UK found that there was no association between Guillain- Barré syndrome (GBS) and seasonal influenza vaccines [16].

Anaphylaxis, angioedema, bronchospasm and urticarial (allergic reactions) can rarely occur, usually due to hypersensitivity to egg protein [1].

First Published :   12 Sep 2017
Last Updated :   12 Sep 2017

  1. Public Health England, Immunisation against infectious disease Chapter 19 Influenza (updated 28 August 2015). [Accessed 7 September 2017]
  2. World Health Organization Fact Sheet: Influenza (seasonal) November 2016 [Accessed 7 September 2017]
  3. World Health Organization, What is a Pandemic, 24 February 2010. Accessed 7 September 2017]
  4. World Health Organization. Transcript of statement by Margaret Chan, Director-General of the World Health Organization. 11 June 2009. [Accessed 7 September 2017]
  5. World Health Organization. H1N1 in post-pandemic phase. 10 August 2010. [Accessed 7 September 2017]
  6. World Health Organization. Recommendations on the composition of influenza virus vaccines. [Accessed 7 September 2017]
  7. Goeijenbier M. van Genderen, P. Ward, B. J. Wilder-Smith, A. Steffen R. Osterhaus, A. D. M. E. Travellers and influenza: risks and prevention. J.Trav. Med. 2016, 1–10
  8. Boggild AK, Castelli T, Gautret P et al. Vaccine preventable diseases in returned international travelers: results from the GeoSentinel Surveillance Network. 28;7,389-95,2010
  9. Marti F, Steffen R, Mutsch M. Influenza vaccine: a travelers’ vaccine? Exp Rev Vac. 7:679-687, 2008
  10. Regan J.J., Tardivel K., Lippold S.A., Kornylo Duong K. Cruise ship travel. In: US Centers for Disease Control and Prevention. Health Information for International Travel 2018, Chapter 6 [Accessed 7 September 2017]
  11. Pebody RG, McLean E, Zhao H, et al. Pandemic Influenza A (H1N1) 2009 and mortality in the United Kingdom: risk factors for death, April 2009 to March 2010. Euro Surveill 2010;15:pii=19571 [Accessed 7 September 2017]
  12. World Health Organization. Pandemic (H1N1) 2009: frequently asked questions. [Accessed 7 September 2017]
  13. Public Health England. Guidance on use of antiviral agents for treatment and prophylaxis of seasonal influenza. October 2016 [Accessed 7 September 2017]
  14. Department of Health, Public Health England, NHS England, The national flu immunisation programme 2017/18. [Accessed 7 September 2017]
  15. Gagnon R, Primeau MN, Des Roches A et al. Safe vaccination of patients with egg allergy with an adjunvanted pandemic H1N1 vaccine. J Allergy ClinImmunol. 126:317-23, 2010
  16. Stowe J, Andrews N, Wise L et al. Investigation of the temporal association of Guillain-Barré syndrome with influenza vaccine and influenza-like illness using the United Kingdom General Practice Research Database. Am J Epidemiol. 169:382-8, 2009


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