Influenza - the Disease


What is influenza?

Influenza, often referred to as 'the flu', is a respiratory infection caused by a member of the influenza virus family. Influenza virus infections are usually more severe than other respiratory virus infections, and typically involve a combination of respiratory (cough, sore throat) and constitutional (fever, headache, muscle aches) symptoms. In older adults and people with certain pre-existing medical conditions, influenza infections can lead to serious and even life threatening complications. A notable feature of influenza is that repeated infections can occur throughout life.

How is influenza spread?

Influenza is transmitted by tiny droplets of moisture from the respiratory tract of infected people spread by coughing, sneezing, touch or even talking. When these are breathed in by a susceptible person, the viruses they contain can enter the cells of the respiratory tract and multiply. The person will usually become ill within 2–3 days but may be contagious and start shedding virus for up to a day before symptoms are noticed.

When is the influenza season?

In temperate regions of the northern and southern hemispheres, the main influenza season falls around the winter months (November – April for northern hemisphere, May – October for southern hemisphere) with sporadic infections at other times. In regions closer to the equator, influenza may occur throughout the year and some tropical countries may experience two peaks of activity annually.

Who is at greatest risk from influenza?

Older adults (in most countries defined as 65 years and above), people with chronic cardiac, respiratory, kidney, metabolic (eg. diabetes) and immune system diseases, pregnant women, and people in long-term care have a higher risk of severe influenza. Infants under the age of 6 months may also be at increased risk but cannot be protected by current vaccines.

More information:

  • Australian Government Department of Health and Ageing
  • US Centers for Disease Control and Prevention (CDC)
  • US National Institute of Allergy and Infectious Diseases
  • WHO Fact Sheet
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    Influenza Biology

    Haemagglutinin structure

    What are influenza viruses?

    Two types of influenza virus, designated type A and type B, are of particular public health concern. They can be readily distinguished from each other by laboratory tests but not by clinical symptoms.Type C influenza viruses are of limited public health significance.

    Influenza viruses are coated with two proteins, called haemagglutinin (HA) and neuraminidase (NA). There are many subtypes of influenza A with different variants of haemagglutinin and neuraminidase, although most of these subtypes only circulate in birds. The subtypes of influenza A currently circulating in the human population are A(H1N1)pdm09 and A(H3N2).

    Influenza B viruses only infect humans and are not classified into subtypes. Two closely related lineages of type B viruses (B/Victoria and B/Yamagata) are currently circulating in humans.

    Occasionally a new subtype of influenza A virus emerges that has a haemagglutinin to which most humans have not previously been exposed – this is known as 'antigenic shift'. In addition to 'antigenic shift', the haemagglutinin and neuraminidase of type A and B viruses continually change by mutation to produce new strains of the virus, a process referred to as "antigenic drift".

    Influenza mutations

    Can I build long-term immunity to influenza?

    Infection with influenza virus, or immunisation, induces the body to produce antibodies to the strain(s) of virus encountered. Antibodies against the haemagglutinin are most important for protecting us against influenza virus infection. As the circulating viruses change by mutation, these antibodies become less effective at neutralizing the virus, and within a relatively short time they are no longer protective. Therefore, infections can occur repeatedly throughout life, and protection by vaccination requires annual dosing with updated vaccines. When a new subtype of influenza A emerges, most of the population is highly susceptible and infection rapidly spreads worldwide, producing a pandemic.

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    Treatment of Influenza

    HI Assay pipetting

    Can influenza be treated?

    There are two forms of treatment for influenza- symptomatic and antiviral. Drugs that lessen the symptoms of the disease, such as a runny nose, high temperature, coughs and headache, are available over the counter in pharmacies and sometimes supermarkets. These have no effect on the course of the infection.

    Other drugs that limit the multiplication of the virus are generally only available by prescription from a doctor. This second group comprises the drugs known as 'neuraminidase inhibitors', zanamivir (Relenza) and oseltamivir (Tamiflu) which act on both influenza A and B, and the older drugs Amantadine and Rimantadine. Amantadine and Rimantadine are no longer recommended as currently circulating influenza strains have developed resistance to this class of drugs. Not all anti-viral drugs are available in all countries.

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    Vaccination against influenza

    HI Assay robot

    How can I protect myself from flu?

    Yearly vaccination is the best way to protect yourself from seasonal influenza infection. In temperate climates this should be undertaken in autumn so that antibodies are at peak levels during the influenza season.

    What is in the influenza vaccine?

    Most influenza vaccines are prepared from influenza virus that has been inactivated and has had some components removed, which means that the vaccines cannot cause infection. When injected, the body's immune system recognizes the fragments of virus as foreign and responds by producing antibodies which will prevent or modify an infection with strains similar to those in the vaccine.

    Quadrivalent influenza vaccines are now currently in many countries, containing the most recent strains of the four types, subtypes or lineages of influenza virus currently circulating globally in humans, A(H3N2), A(H1N1)pdm09 and the two lineages of B viruses (B/Victoria and B/Yamagata). Trivalent influenza vaccines may still be offered in some countries or under specific circumstances (eg. high dose vaccine), containing A(H1N1)pdm09, A(H3N2) and one of the two influenza B lineage viruses.

    Can I get the flu from the flu shot?

    Definitely not. The influenza particles in the vaccine are killed and are not infectious.

    Why do I still get colds after I have had the flu shot?

    The flu vaccine does not protect you from common colds or a number of other respiratory diseases that may circulate during the winter.

    Why do I need to get a flu shot every year?

    This is recommended for two reasons. Firstly, the strains of influenza virus circulating in the community often change from year to year and, secondly, the protection generated after influenza vaccination is relatively short lived, unlike some other vaccinations such as tetanus.

    What about the new intranasal vaccine?

    An intranasal vaccine that uses live attenuated viruses is available in some countries (USA, Canada, UK) but not in others (Australia, New Zealand). This vaccine is used in a more limited age group compared to the traditional vaccine and is especially suited to children aged 2 years and older. The vaccine is administered via a mist into the nostrils instead of via an injection into the arm. This results in a short term low-level infection which may result in a runny nose for a few days.

    More information:

  • Immunise Australia Program
  • Australian Technical Advisory Group on Immunisation (ATAGI)
  • The Australian Immunisation Handbook
  • National Centre for Immunisation Research and Surveillance
  • US Centers for Disease Control and Prevention (CDC) Q&A on vaccine effectiveness
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    Pandemic Influenza

    Influenza Virion Biorender

    What is pandemic influenza?

    An influenza pandemic occurs when a new strain of influenza emerges to which most of the human population has not been previously exposed. This means that immunity within the community is low, allowing rapid infection and transmission of the new strain amongst people on a global scale. Over the course of the past 500 years it is thought that about 13 influenza pandemics have occurred.

    In 2009 the world experienced its first pandemic in 41 years with the emergence of the pandemic A(H1N1) 2009 virus (A(H1N1)pdm09). This virus had many characteristics of viruses circulating in swine and is also related to the 1918 pandemic virus. The A(H1N1)pdm09 virus now continues to circulate in human populations as one of the seasonal influenza viruses.

    Is the 2009 pandemic influenza more dangerous than other types of influenza?

    For most people infected by the A(H1N1)pdm09 virus, symptoms are relatively mild. However, certain population groups are at higher risk for severe illness or complications, including young children (aged under 5), elderly people (aged over 65), pregnant women, people with chronic cardiovascular disease or lung disorders, people with metabolic disorders or immunosuppressive conditions, and indigenous populations.

    Can I be vaccinated against the 2009 pandemic influenza?

    Yes, the current WHO recommended seasonal vaccines for both northern and southern hemispheres contain a A(H1N1)pdm09 component.

    Does the 2009 pandemic mean that there will not be another pandemic soon?

    The timing and severity of pandemics are unpredictable. The key event that leads to a pandemic is a random genetic shuffling which generates a new influenza virus able to spread easily among humans. WHO and national governments of many countries have influenza pandemic emergency plans in place.

    More information:

  • Australian Health Management Plan for Pandemic Influenza (AHMPPI)
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    Avian H5N1, avian H7N9 and other influenza viruses

    Chick Biorender

    What is avian influenza H5N1?

    While only two influenza A subtypes currently circulate in humans, influenza A viruses of all subtypes can infect birds. Of particular concern to public health authorities is the highly pathogenic avian influenza A(H5N1) virus ('bird flu') which causes devastating disease in domestic poultry flocks. H5N1 viruses spread rapidly at a local level amongst poultry and their global transmission has been facilitated by international trade in poultry and poultry products and, to some extent, by migratory birds. Most known cases of human infection with these viruses have occurred in people who have had close contact with infected birds. To date cases of human-to-human transmission have been extremely rare.

    What is the concern about H5N1 influenza?

    Public health authorities are concerned that an H5N1 virus may undergo changes that allow it to spread easily between humans, causing a pandemic. Furthermore, in cases where people have been infected by the H5N1 virus, the disease has been particularly severe and aggressive. If the new virus retained the properties of current H5N1 viruses that cause severe disease in humans, the mortality rate would be high.

    What is being done to prepare for a H5N1 influenza pandemic?

    WHO continues to monitor human cases of H5N1 infection throughout the world, as well as avian outbreaks. WHO Collaborating Centres and other laboratories in the WHO Global Influenza Surveillance and Response System (GISRS) collect and characterise H5N1 viruses isolated from infected humans and birds, and the WHO reports regularly on the availability of suitable H5N1 vaccine candidates that correspond to the different groups (clades and subclades) of H5N1 viruses circulating in different parts of the world. A number of research institutions and vaccine manufacturers are developing vaccines for H5N1 influenza and taking them into clinical trials. Many individual governments also have public health emergency plans should a pandemic occur. In Australia, local bird populations continue to be monitored – to date, highly pathogenic H5N1 viruses have not been found in Australian avian populations.

    What is avian influenza H7N9?

    In 2013 a novel avian influenza A(H7N9) virus emerged in China that has infected humans with a high fatality rate. Unlike highly pathogenic avian influenza (H5N1), avian influenza A(H7N9) does not kill poultry and so is much harder to detect in the environment than H5N1 viruses as it must be detected by laboratory testing. Most known cases of human infection with these viruses have occurred in people who have had close contact with infected birds or contaminated environments. To date, there has only been rare evidence of human-to-human transmission. Currently this virus has been largely confined to China and circulates most prominently from December to April each year. WHO continues to monitor human cases of H7N9, as well as working with animal health organisations to determine the source of the infection and also support the development of possible vaccines for H7N9 influenza.

    Are there other types of non-human influenzas?

    In addition to avian influenzas, influenza viruses can also infect other animals. While birds can be infected by most of the known influenza A subtypes, a smaller number of subtypes are also known to infect pigs, horses, dogs and other species. Recently bats have been found to have influenza-like viruses. Historically there have been no recorded instances of natural transmission from horses or dogs to humans, but transmission from pigs to humans does occur from time to time. The pandemic H1N1 influenza strain that emerged in 2009, commonly referred to as the 'swine flu', was the result of a series of genetic shuffling events between swine, avian and human influenza viruses that had occurred over many years, ultimately producing a virus that is now transmitted readily between people.

    More information:

  • World Health Organisation information about Avian Influenza
  • Australian Government Department of Health
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    Additional Resources