What is influenza?

Influenza (flu) is a contagious respiratory illness caused by infection with an influenza virus. In Europe, influenza occurs in regular annual epidemics in the winter season. These epidemics are associated with high hospitalisation rates and mortality. 

The mortality rate due to influenza has been estimated in the EU/EEA countries participating to the EuroMOMO network (1) to be as high as 25 deaths per 100 000 population in the 2016/27 and 2017/18 influenza seasons (2).

Animals can also be infected with influenza. In some cases, humans can be infected by animals, such as in cases of avian and swine influenza.

In addition to seasonal epidemics, novel influenza viruses may occasionally emerge and cause pandemics. A pandemic is the rapid spread of a new human influenza around the world. The 2009 A(H1N1)pdm09 influenza pandemic accounted for 123 000 to 203 000 deaths worldwide and a significant global socio-economic burden (3).

Influenza pandemics happen when a new strain of a flu virus appears which can infect humans, to which most people have no immunity and which can transmit efficiently from human to human.

Symptoms of influenza

What are the symptoms of influenza?

Not everyone who is infected with an influenza virus becomes unwell. For those that do, common symptoms include:

  • fever or feverishness;
  • headache;
  • muscle pain;
  • general feeling of ill-health;
  • runny nose;
  • sore throat;
  • non-productive cough.

The severity of the disease varies widely, from no symptoms at all through to severe illness. In uncomplicated cases, symptoms resolve spontaneously within one week from onset.


Father and child

What are the complications of influenza?

Complications of influenza include pneumonia and encephalitis (inflammation of the brain). People with pre-existing medical conditions are at higher risk of their condition worsening following influenza infection.

How is influenza spread?

Influenza is easily transmitted from person to person. This is mainly via direct contact with secretions from infectious patients, such as droplets of fluid released when they cough or breathe out. It is also spread via secretions on hands, tissues and surfaces that people touch.

Who is at risk of influenza?

Approximately 20% of the population gets infected with influenza each year and one in four infected people will develop symptoms. Children are infected slightly more often than adults.

Severe illness and complications are more common in very young infants, the frail elderly and certain medical risk groups. Even so, about half of the children and working-age adults needing admission to intensive care have no pre-existing medical conditions.

process of vaccination illustration

How can influenza be prevented?

Vaccination is the most effective way to prevent influenza. The EU Member States recommend seasonal influenza vaccination for risk groups, such as the elderly, as well as individuals with chronic medical conditions like heart disease, problems with the lungs and airways, diabetes or immune system problems.

The World Health Organization (WHO) also recommends vaccination for healthcare workers. The EU Council Recommendation on seasonal influenza vaccination encourages countries to improve vaccination coverage among healthcare workers.

Most EU Member States follow the WHO recommendations to vaccinate pregnant women, and some follow recommendations to vaccinate healthy children aged 6 - 59 months.

An update of seasonal influenza vaccines is needed yearly, since influenza viruses constantly evolve.

In addition to vaccination, antiviral medicines have been shown to be safe and effective measures to prevent influenza infection or to reduce the severity of infection in certain settings, such as nursing homes.

How is influenza treated?

For most cases of influenza, patients are advised to take bed rest and drink plenty of fluids in order to manage their symptoms; staying at home also minimises the risk of infecting others. Patients are advised to seek medical advice if their condition continues to get worse.

Antiviral medicines, taken as rapidly as possible after illness onset, are safe and effective measures for the treatment of influenza. However, antivirals are generally recommended for severe or rapidly progressing cases. In some cases patients develop bacterial complications following influenza infection and need to be treated with antibiotics.


(1) EuroMOMO - European monitoring of excess mortality for public health action:

(2) Nielsen J. et al. European all-cause excess and influenza-attributable mortality in the 2017/18 season: should the burden of influenza B be reconsidered? Clinical Microbiology and Infection. 2019; Volume 25, Issue 10, 1266 – 1276. Available from:

(3) Simonsen L. et al. Global mortality estimates for the 2009 Influenza Pandemic from the GLaMOR project: a modeling study. PLoS Med. 2013 Nov; 10(11): e1001558. Available from:

For more information check the ECDC website:

Note: The information contained in this factsheet is intended for the purpose of general information and should not be used as a substitute for the individual expertise and judgement of a healthcare professional.

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