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 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 As these different viruses are constantly mutating, the immunity we gained from previous exposure is gradually outdated.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.
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.
Infection control measures
Hand and cough hygiene
In the case of new respiratory tract symptoms
- Influenza vaccine - Theme page for information, news and updated advice about the annual influenza vaccine.
Influenza vaccine for risk groups
Influenza in Norway
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.