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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 lost.

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.

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 new 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. Subsequent to the 2009-2010 pandemic period, this variant has become established as one of the regular seasonal influenza viruses. The virus 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 circulates, 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 trasmitted by contact.

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. 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, malaise 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

The 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. In years with larger epidemics, 10 to 30 per cent of the Norwegian population may be infected. During the Hong Kong influenza outbreak in 1970, an estimated 15-40 per cent of the population became ill.

Estimates made in Norway 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.

Sykdomspulsen influensa ukerapport.
Sykdomspulsen influensa ukerapport.
Figure 1. Influenza-like illness reported in Norway 2006- 2014. The curves show the percentage of physician consultations where the patients have been diagnosed with influenza-like illness from week 20 in autumn to week 20 in spring. The yellow curve shows the pandemic in 2009.

Treatment of influenza

Treatment with antiviral drugs such as zanamivir (Relenza) and oseltamivir (Tamiflu) may shorten the illness by 1 to 2 days and reduce symptoms. This treatment is prescribed and to be effective it must begin no later than 48 hours after the first symptoms appear.

Antiviral medicines are also approved as a preventive treatment where there is evidence of influenza in the community; oseltamivir from 1 year of age and zanamivir from 5 years of age. Preventive treatment of influenza is rarely done in Norway.

These medicines are only effective only against influenza. The medicines are recommended for use in influenza disease in the risk groups (see below), but also from clinical assessment in other groups.

In most cases of influenza, it is sufficient to treat the symptoms and not the virus infection itself. Paracetamol (acetaminophen) is a prescription-free drug that acts as both an antipyretic (reduces fever) and analgesic (painkiller). Aspirin can also be used but is not recommended for children under 12 years old. If a bacterial infection is also present, antibiotics may be needed.

Preventive measures

The general prevention advice is to wash hands often and avoid coughing and sneezing on others. If you have a fever, you should stay away from work, school and child care centres, both to recover and to avoid infecting others. Children can return to child care when they feel better.

The most important preventive measures are vaccination and medication. The seasonal influenza vaccine contains the influenza virus variants recommended by the WHO and should be given between September and November. Protection is acquired after one to two weeks. The vaccine provides protection in 50-80 per cent of those vaccinated. The degree of protection depends on the person’s age and whether there is a good match between the circulating viruses and the virus used in vaccine production. In the elderly, protection against becoming ill is somewhat lower, but the vaccine reduces the risk of complications.

Every year, the Norwegian Institute of Public Health (NIPH) distributes approximately 400,000 doses of seasonal influenza vaccines to groups who are recommended to take the influenza vaccination.

The influenza vaccine is especially recommended for people in these groups:

  • Pregnant women after week 12 of pregnancy (2nd and 3rd trimester).
    Vaccination of women in their 1st trimester can be considered if they have additional risk factors for complications from influenza.
  • Residents in nursing homes and sheltered accommodation
  • Everyone 65 years old or above
  • Children and adults with:
    • diabetes type 1 and 2
    • chronic respiratory disease
    • chronic cardiovascular disease
    • chronic liver failure
    • chronic renal failure
    • chronic neurological disease or injury
    • immunodeficiency disorders
    • severe obesity (BMI over 40)
    • other severe or chronic illness evaluated on an individual basis by a doctor

In addition, the influenza vaccine is recommended to the following groups:

  • Health professionals with patient contact. They are largely exposed to infection and if they become infected they can infect their patients.
  • Household contacts of highly immunosuppressed patients should consider vaccination against influenza to protect the patient.
  • Pig farmers and others who have regular contact with live pigs. The main reason to vaccinate pig farmers is to protect pigs against influenza infection, thus preventing the development of new viruses among pig herds.

The risk groups and other groups who are recommended to have influenza vaccination can receive the vaccine at a reduced price by contacting their physician or any municipal vaccination clinic.

The risk groups can receive the seasonal influenza vaccine combined with the vaccine against pneumonia. Unlike the seasonal influenza vaccine, the pneumococcal vaccine is not given annually. The elderly are recommended to be revaccinated after about 10 years. Some risk groups are recommended more frequent revaccination.

All vaccinations, including influenza vaccine, are recorded in the Norwegian Immunisation Registry (SYSVAK) if the patient has given consent.

With outbreaks in health care institutions, health care workers should follow special guidelines.

Notification duty

Influenza outbreaks in hospitals and health care institutions should be notified to the NIPH according to defined rules.


The World Health Organization (WHO) has established a network of national influenza centres in 110 countries to monitor influenza activity and to recommend the composition of next season's vaccine.

Influenza surveillance at the NIPH

The NIPH is responsible for monitoring influenza in Norway. The institute monitors which influenza viruses are circulating in the population and compiles statistics.

The Norwegian Syndromic Surveillance System (NorSSS) is a new automated electronic system that weekly provides data about the occurrence of influenza-like illness in each county. Data from all general practitioners and emergency clinics is collected.  The previous surveillance system based on data from 201 sentinel physicians’ offices, known as watchtowers, has now been replaced by NorSSS. In addition, a number of physician offices provide specimens from patients with influenza-like illness for testing in the NIPH's National Influenza Centre, and other laboratories submit samples containing influenza virus for further analysis. This information is collected in many countries and is used both for international monitoring and the production of next year's influenza vaccine.

In order to monitor immunity in the population, blood samples are collected annually during the summer and analysed in the National Influenza Centre. These provide information about the population’s antibodies against different influenza virus variants and indicate which groups were particularly affected. This also gives a picture of immunity against the viruses that are expected to be circulating during the upcoming season.

If you have any questions about influenza, consult your physician.