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.

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 As these different viruses are constantly mutating, the immunity we gained from previous exposure is gradually outdated.

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.

Influenza symptoms

Typical influenza symptoms appear suddenly with:

  • fever
  • muscle aches
  • headaches
  • 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.

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.

Who should be vaccinated?

Almost 1.6 million people in Norway belong to a group with increased risk of a severe influenza disease course.

People in the following risk groups should have the influenza vaccine annually:

  • Residents of nursing homes and care homes
  • Everyone aged 65 years or over
  • Pregnant women from week 12 of their pregnancy (2nd and 3rd trimester). Pregnant women in their first trimester with an additional risk should also be offered the influenza vaccine.
  • Children who were premature at birth, particularly those born before week 32 of pregnancy, from 6 months old (chronological age) to 5 years 
  • chronic lung disease
  • chronic cardiovascular disorders (other than well-managed high blood pressure)
  • diabetes, types 1 and 2
  • chronic liver failure or chronic renal failure
  • chronic neurological disorders or injury, particularly people with reduced lung capacity and/or coughing strength
  • impaired immune function as a result of disease or treatment (e.g. organ transplantation, cancer, HIV, rheumatoid arthritis and other diseases)
  • morbid obesity (BMI over 40)
  • other serious or chronic diseases where influenza is a serious health risk, after individual assessment by a doctor (e.g. people with congenital chromosomal abnormalities, genetic syndromes and complex chromosomal abnormalities not classified elsewhere)

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.


22 79 90 00

Medlemsorganisasjonen LHL jobber for mennesker med hjerte-, kar- og lungesykdom, allergi, hjerneslag, afasi, og deres pårørende.

Content provided by Norwegian Institute of Public Health

Norwegian Institute of Public Health. Influenza. [Internet]. Oslo: The Norwegian Directorate of Health; updated Thursday, November 17, 2022 [retrieved Thursday, June 20, 2024]. Available from:

Last updated Thursday, November 17, 2022