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Factsheet for the general public

It is for information purposes only. It is not medical advice. If you need advice on health matters you should contact healthcare in your country.
Influenza is a contagious disease caused by the influenza virus. Influenza viruses are divided into three types: A, B and C. There is also evidence of a fourth type: D. Both influenza A and B circulate each season. One of the most distinctive properties of influenza A and B viruses is that they change continuously. These changes mean that previous influenza infection or vaccination may not protect against a new influenza infection.
People who catch influenza mostly have fever and respiratory symptoms.
In Europe, and the rest of the northern hemisphere, influenza occurs every winter. However, the impact of influenza varies from year to year. Usually, winter epidemics affect each of the countries for one to two months and, in Europe as a whole, last for about four months.  Sporadic infections can also occur outside of the influenza season. In the southern hemisphere an epidemic occurs in its winter (June to October) and in some countries close to the equator there is year-round circulation of seasonal influenza.
For most people influenza is just an unpleasant infection. But according to a rough estimate, up to approximately 70 000 persons in the EU and EEA countries could develop more serious illness and die each year from influenza. Influenza also causes considerable costs to the health services. In addition, large numbers of milder cases need time off work. However influenza is a vaccine-preventable disease and some of its impact can be avoided.
Although influenza A and B viruses change over time, it is possible to produce an updated vaccine for the coming influenza season with a good likelihood that it will be protective. This is because the viruses causing the epidemic are usually fairly similar to the previous year's circulating viruses.
Some people are more at risk of becoming seriously ill from influenza, for example those with long-lasting ill-health and older people. Therefore the World Health Organization (WHO) recommends they get vaccinated yearly against influenza. In Europe vaccination is carried out in the autumn/beginning of winter.


Seasonal influenza virus spreads fairly easily from one person to another. The virus spreads mostly by droplets when people cough and sneeze, and by respiratory secretions getting onto hands and tissues.

Influenza symptoms are:

  • sudden onset of fever,
  • runny nose,
  • sore throat,
  • cough,
  • a general feeling of ill-health,
  • headache,
  • aching muscles,
  • children may also vomit or have diarrhea, apart from fever and respiratory symptoms.

But not everyone with influenza will necessarily show all symptoms. The more serious symptoms typically last only for a few days. The sore throat and runny nose may last longer. Cough and a general feeling of ill-health may last some weeks. Sometimes people can be infected with influenza but have very mild symptoms or no symptoms at all. But influenza often makes people quite ill and it takes a week or so to recover. It should also be noted that many other viruses and bacteria can cause symptoms similar to influenza. Therefore not everyone with flu-like symptoms has influenza.

In some cases the disease becomes more severe. This can be due to wider spread of the virus in the body or a second, usually bacterial, infection, e.g. severe lung infection which can be deadly. It can also result in inflammation of the heart muscle or inflammation of the brain which can also be deadly. The more severe illnesses are more common in the elderly, the very young, and those with other long-lasting medical conditions like diabetes or heart disease. However, severe infections can occur in people in good health of any age group. Influenza can also lead to worsening of underlying disease e.g. cardiac or pulmonary disease.
Once you have been infected with a particular influenza virus you usually become immune to it. This means that the defense system of the body prevents you from falling ill again from the same influenza virus. You can also get immune to a particular influenza virus by being vaccinated. However, the circulating influenza virus changes from year to year and the vaccine protection wanes. Therefore the immunity tends to last only for one winter. That is why people who are recommended influenza vaccination need to be vaccinated every year before the influenza starts spreading.
The number of days from when you come across a germ until you develop symptoms is called incubation time. For seasonal influenza this time ranges from one to four days, but the average is two days.
Influenza type A and B are of more concern as they cause more serious illnesses.
At irregular intervals there are influenza pandemics. During these pandemics other population groups may be affected by severe illness than during seasonal influenza epidemics. 


Medical advice must be sought quickly if symptoms become severe or last more than about a week.

There is specific antiviral medication against influenza (the neuraminidase inhibitors oseltamivir and zanamivir). The policy and practice on the use of antivirals varies between European countries. The scientific evidence suggests that antivirals shorten the course of the disease if they are taken early on in the illness, preferably within 48 hours. The earlier the better.


Vaccines against influenza are the most effective form of protection against influenza.
Although severe illness and complications from flu can occur in anyone, they are far more common among the risk groups. For people in a risk group it is important to prevent serious complications of seasonal influenza. Prevention can be achieved by receiving the annual flu vaccine before the seasonal influenza starts to circulate.
The WHO recommends seasonal influenza vaccination for the following risk groups: pregnant women, children aged 6-59 months, the elderly, and individuals with long-lasting medical conditions. Examples of long-lasting medical conditions are diabetes, immunosuppression, neurologic or neuromuscular disorders, or cardiac and respiratory disease. WHO also recommends vaccination of healthcare workers.
A European Council recommendation supports reaching a high level of influenza vaccination coverage among risk groups. However, the implementation varies from country to country.
For current recommendations on target groups for influenza vaccine, please check with your national public health agencies.



The vaccine contains three or four components selected to protect against the main groups of circulating influenza viruses.
Yearly vaccination is recommended because the circulating flu virus strains evolve from year to year and the vaccine components are adjusted accordingly. Even if the vaccine does not offer full protection, it is still of value to be vaccinated if you belong to one of the groups that are recommended influenza vaccination.

Seasonal influenza vaccine has been used for decades and we know that seasonal influenza vaccines are among the safest vaccines around.


Although vaccination is the first option for preventing influenza, antiviral medication can also be useful. Since the 1990s, antiviral drugs have been demonstrated to be effective both for treatment and protection against influenza. However, the policies and use of antiviral drugs vary between European countries.

Apart from vaccination and antiviral medication, there are a number of measures that people may take to reduce the risk of seasonal influenza transmission. The following suggestions are based in part on studies and in part on judgement based on public health experience, and apply regardless if you fall ill or want to reduce the risk of falling ill:
  • The infectious period varies somewhat from person to person. Otherwise healthy adults with seasonal influenza start being able to pass on the infection from the day before the symptoms appear. They then remain potentially infectious for five to seven days. Infectiousness is highest during the first three to five days of the illness. This is why it is important, especially when influenza is present in the community, that when someone starts feeling ill they go home and stay home for a few days.
  • If the infected person coughs or sneezes and doesn’t cover his or her mouth and nose people within two meters can be infected. That is why it’s important that a person covers his or her mouth and nose (preferably with a disposable tissue) when he or she sneezes. Tissues should be thrown in the bin after use.
  • Influenza infection can also be spread by direct contact of the mucous membranes of the nose, mouth and throat with virus, for example from the hands of infectious people who have rubbed their noses. That is why frequent proper hand-washing and use of disposable handkerchiefs are considered to have the potential to reduce the risk of catching influenza. Hands should be washed thoroughly with soap and water. After washing with soap and water, dry hands thoroughly with single-use paper towels. Hand-washing with drying of hands should last for at least 40-60 seconds each time.
    Alcohol-based hand sanitisers reduce the amount of influenza virus on contaminated hands. When hand-washing is not possible, alcohol-based hand sanitisers are an option.
  • Avoid close contact with sick people. Maintain a distance of at least one meter from someone with symptoms of influenza, and avoid crowded situations. When distance cannot be maintained, reduce the time of close contact with people who might be ill.
  • Avoid touching your eyes, nose or mouth. Viruses are often spread when a person touches something that is contaminated with germs and then touches his or her eyes, nose, or mouth.
© European Centre for Disease Prevention and Control (ECDC) 2005 - 2017