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  • Influenza - Fact sheet about seasonal influenza

Fact sheet

Influenza - Fact sheet about seasonal influenza

Influenza causes fever and body aches as well as respiratory symptoms. Typically, five to ten per cent of the population becomes ill during a winter season. Influenza seasons vary in severity.


Influenza causes fever and body aches as well as respiratory symptoms. Typically, five to ten per cent of the population becomes ill during a winter season. Influenza seasons vary in severity.

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Influenza outbreaks can cause epidemics with many serious ill or dead, and are therefore closely monitored. In addition, extraordinarily large epidemics, influenza pandemics, occur every few decades.

What is seasonal influenza?

Every winter, the population in the northern hemisphere is affected by influenza outbreaks. This is called seasonal influenza. Two types of influenza virus, types A and B, are the cause of these outbreaks. As these different viruses are constantly mutating, the immunity we gained from previous exposure is gradually outdated.

Influenza pandemics in history

Epidemics probably caused by influenza are known far back in time. It is especially the extraordinary, large epidemics that have been recorded. A total of 19 such large influenza epidemics, called pandemics, have been recorded in history. Only influenza A can cause a pandemic.

The first pandemic was described in 1580. The largest pandemic was the Spanish influenza of 1918-19, which resulted in 25-40 million deaths worldwide. In Norway, about 15 000 people died.

The last three major pandemics were Asian influenza in 1957, Hong Kong influenza in 1968 and the H1N1 pandemic in 2009/2010.

The variant of A (H1N1) virus that caused the pandemic in 2009/2010, is often called "swine flu" since it is closely related to the classical swine influenza virus. This variant has become established as one of the regular seasonal influenza viruses and is now officially called A(H1N1)pdm09.

Immunity and influenza vaccine

Influenza virus type A was first discovered in 1933 and type B in 1940. In the 1940s, the first influenza vaccine was developed, but it soon became clear that the virus mutated over time and the vaccine had to be adapted to the current circulating virus variants.

Influenza viruses have since been carefully mapped, with their characteristics, evolution and ecology. While influenza type B is almost exclusively found in humans, influenza virus type A is found in other animals, particularly in aquatic birds. Influenza A is further divided into many subtypes that have different surface molecules. Immunity against one subtype does not protect much against a virus of a different subtype.

Two main lineages of influenza B are circulating, called B-Victoria and B-Yamagata.

Droplet and contact transmission

Influenza is transmitted by droplets, aerosols or contact. Droplets and aerosols containing influenza virus can be inhaled. These are transmitted when infected people cough or sneeze. The droplets can adhere to surfaces, whereby influenza can be transmitted by contact. A small viral dose is enough to cause disease.

The incubation period from infection to symptoms is usually two days, but this can vary from one to four days. A person is infectious from the day before symptoms appear and for three to five days afterwards. It is possible to be infected even without symptoms. The relatively high infection rate means that epidemics can develop rather quickly.

After being ill with one influenza virus strain, immunity against that strain usually lasts for many years. Cross immunity to similar strains is also possible. The ability to develop immunity varies with age.

Influenza symptoms

Typical influenza symptoms appear suddenly, with fever, muscle aches, headaches and malaise. In addition, there can be respiratory tract symptoms such as runny nose, sore throat and dry cough. Diarrhoea and vomiting is rare among adults, but may occur in children. Influenza usually lasts for seven to ten days.

In addition to the viral disease, influenza can cause complications such as bacterial infection, for example pneumonia, sinusitis or otitis.

Many of those that get infected have few or no symptoms, but can still infect others.

Detection of influenza

An influenza diagnosis is most often a clinical diagnosis, based on symptoms. It is possible to take a test to verify the diagnosis. The test is taken from the nose or throat, and should be taken early in the course of the disease.

Prevalence of seasonal influenza

The timing and size of the outbreak varies widely between the different winter seasons and depends on the immunity in the population. In years with larger epidemics, 10 to 30 per cent of the Norwegian population may be infected. Outbreaks usually start around Christmas/New Year and last for about 12 weeks. The peak in recent seasons has fallen around New Year or early February. However, this can vary between seasons.

Estimates made in Norway in the period 1975-2004 suggest that about 900 deaths a year are caused by influenza. However, this is only an average and can vary greatly from year to year. The people who die are most often elderly and or have chronic underlying diseases.

Treatment of influenza

For most cases of influenza, treatment is intended to alleviate symptoms. Symptoms can be reduced with fever-reducing and pain-relieving drugs such as paracetamol. Children under the age of 12 with influenza infection should not be treated with acetylsalicylic acid (Dispril, Aspirin). Ear infections and pneumonia are examples of conditions that can arise. A doctor can then assess whether antibiotics are necessary.

The doctor must assess whether patients with influenza, regardless of whether they have been tested or vaccinated, should be offered antiviral treatment. The assessment must be based on the extent of symptoms, whether the patient belongs to a risk group, how long the patient has been ill, whether there are special infection control considerations, and the patient's own wishes. Treatment should start within 48 hours of the onset of symptoms, but seriously ill patients can also benefit from treatment if started later.
In some cases, it may be appropriate to use antiviral drugs as preventive treatment after exposure to influenza infection. This may be appropriate in nursing homes, or in households where a member of the household has a weakened immune system.

Preventive measures

This autumn and winter, there is common infection control advice against influenza, coronavirus and other types of respiratory tract infections.

Infection control measures

If possible, close contact with sick people should be avoided.

Hand and cough hygiene

Practise good hand hygiene and keep your hands clean. Use soap and water or an alcohol-based hand disinfectant. Use a tissue or the crook of your elbow when you cough or sneeze. Throw away the tissue carefully and wash your hands afterwards.

In the case of new respiratory tract symptoms

Stay at home if you are ill or have new respiratory symptoms (fever, cough, shortness of breath, headache, weakness, muscle pain, sore throat, etc.). Limit contact with others until you feel well again, especially people at risk of complications from influenza.


Vaccination is the best preventive measure we have against influenza. The seasonal influenza vaccine contains the influenza virus variants recommended by the World Health Organization (WHO). The vaccine should be given in October-December.
  • Influenza vaccine - Theme page for information, news and updated advice about the annual influenza vaccine.
The Norwegian Institute of Public Health recommends the influenza vaccine for people with an increased risk of a severe disease course. The vaccine is also recommended for employees in the health service and those close to those who are immunosuppressed.

Influenza vaccine for risk groups

Many of those who are recommended a vaccine against seasonal flu are also recommended a vaccine against serious pneumococcal disease, the so-called pneumococcal vaccine.

Influenza in Norway

The NIPH monitors influenza in Norway, which is important to be able to assess the severity of outbreaks. A continuous overview is kept of which influenza viruses are circulating in different parts of the country, and whether the virus is changing.
An overview is also kept of how many people see a doctor due to an influenza-like illness, how many are in hospital and intensive care units, as well as whether excess mortality occurs in the population.


The World Health Organization (WHO) has established a network of national influenza centres to monitor influenza activity and to recommend the composition of next season's vaccine.

The NIPH contributes data to various international monitoring systems, including to the WHO. Data from Norway is important for the WHO's work with the preparation of new influenza vaccines and helps ensure that new viruses can be detected and notified quickly.