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What is an influenza pandemic?

A pandemic is a worldwide outbreak of a disease. A pandemic of influenza occurs when a new influenza A virus emerges which is able to cause illness in humans, and which can readily spread from one person to another.

Pandemic influenza viruses are sufficiently different from the circulating seasonal influenza viruses that few if any people in the community have immunity against it, including those people who have had seasonal flu or a seasonal flu vaccine before.

How is an influenza pandemic declared?

The World Health Organization (WHO) has a set of pandemic phases that it uses to describe the global situation (phases 1-6). The WHO Director-General is responsible for declaring changes to the global pandemic phase, following advice from an international expert advisory group. Phase 5 is characterized by human-to-human spread of a novel influenza virus into at least two countries in one WHO region.

While most countries will not be affected at this stage, the declaration of Phase 5 is a strong signal that a pandemic is imminent. Phase 6, the pandemic phase, is characterized by community level outbreaks in countries in different WHO regions. Designation of Phase 6 indicates that a global pandemic is under way. Australia and other countries are likely to implement measures to reduce the impact of a potential pandemic influenza strain well before WHO formally declares a new pandemic.

The Australian Health Management Plan for Pandemic Influenza describes the response stages to a new influenza pandemic in Australia, how they are declared and how they relate to WHO's pandemic phases.

Where do pandemic influenza viruses come from?

Small mutations regularly occur in the surface proteins of influenza A viruses creating new variations. This is called antigenic drift and means seasonal influenza vaccines often need to be modified each year to better match the circulating viruses.

Unpredictably, entirely new influenza A subtypes can emerge with the capacity to infect humans. This can come as a result of a large mutation in a non-human influenza A virus (such as an avian or swine influenza virus), called antigenic shift. It can also occur when the genes of two or more influenza A viruses mix to produce a new virus. This is called gene re-assortment.

For an influenza pandemic to occur, three criteria must be met:

  • a new influenza virus emerges to which humans have little or no immunity
  • the new virus must be virulent enough to cause disease in humans
  • the new virus must have the capacity to spread efficiently from person to person.

What is the impact of an influenza pandemic likely to be?

The impact of an influenza pandemic depends on how infectious the virus is and how sick it makes people (the virulence of the virus). The most severe influenza pandemics are associated with a new influenza virus that spreads very easily between people and also causes severe illness. Pandemic influenza viruses that tend to cause milder illness can still have a major impact as most people in the community will be susceptible to infection.

The consequences of an influenza pandemic could include:

  • Many people may become unwell and some will die.
  • People who are sick with pandemic influenza, or who have been in close contact with sick people, may be asked to stay at home.
  • Many people may be unable to work due to changes in work practices or temporary work closures.
  • Many people will need to stay at home to care for others who are sick.
  • Public services such as power, water and transport and other services may be disrupted.
  • Supplies of basic goods to shops may be disrupted or delayed.
  • Schools, businesses and entertainment venues may temporarily close.
  • Health care may be delivered differently and health care resources may be stretched.

Three influenza A pandemics are known to have occurred during the 20th century: in 1918-1919, 1957-1958 and 1968. Of these, the 1918-1919 pandemic (sometimes called the ‘Spanish flu”) was the most devastating. At its peak it incapacitated cities and paralysed healthcare systems in many countries, infecting one quarter of the world’s population and causing at least 50 million deaths worldwide.

The first pandemic of the 21st century began in 2009 and was caused by what has become known as the pandemic (H1N1) 2009 influenza virus. While the overall death rate of this virus was similar to seasonal influenza, the disease was far more likely to affect healthy adolescents and young adults than seasonal influenza. While people in older age groups were relatively spared, the demands on the healthcare system were still well beyond those of a typical winter influenza season.

What is swine flu?

The term ‘Swine flu’ refers to influenza viruses that have adapted to spread amongst pigs. Pigs are able to be co-infected with avian (bird), swine and human viruses. Occasionally swine influenza viruses can be transmitted to humans but they are usually not able spread between people.

The virus that caused the influenza pandemic in 2009 was incorrectly labelled as ‘swine flu’. This is explored in What caused the influenza pandemic in 2009?

What caused the influenza pandemic in 2009?

The influenza pandemic that began in 2009 was caused by a new type of influenza A virus, now called pandemic (H1N1) 2009. This virus was first detected in Mexico and the United States in April 2009.

This virus was a unique new combination of influenza virus genes never previously identified in either animals or people. The virus genes were a combination of genes most closely related to North American and Eurasian swine influenza viruses. This is why it was popularly known as ‘swine flu’ even though it was not a virus known to have circulated among pigs.

The new virus was very different to the seasonal human H1N1 influenza virus that had been circulating in recent years, although experts believe it shares some characteristics with seasonal H1N1 influenza strains that circulated in the past. This is supported by the observation that older people had lower rates of infection overall during the pandemic, and studies have shown that people in older age groups are more likely to have evidence of immune protection against the pandemic strain.

How did NSW respond to the 2009 influenza pandemic?

The first reports from Mexico of a novel influenza strain came on 24 April 2009. NSW Health immediately advised Emergency Departments and General Practitioners to be on the lookout for possible cases in NSW. What began primarily as a public health response gradually grew to encompass many areas of the NSW Health system and the community. NSW was also active in the process of planning a coordinated national response to the influenza pandemic threat.

The World Health Organization (WHO) raised its pandemic alert level to phase 4 on 27 April. On the same day, Australia moved to the DELAY pandemic phase under the Australian Health Management Plan for Pandemic Influenza (AHMPPI). WHO raised the pandemic alert to phase 5 on 29 April, signalling countries to accelerate their preparations for a new influenza pandemic.

In the early stages NSW Health focused on the rapid identification of cases of influenza amongst returning travellers through border screening and assessment of returning travellers from affected areas at international airports. On 20 May, NSW declared its first (and Australia’s second) confirmed case of infection with the new virus in a returned traveller.

Despite intensive control efforts there was increasing evidence of community transmission of the new influenza virus strain within Australia. Recognising this, Australia moved to the CONTAIN phase of the AHMPPI on 22 May. This phase focussed on limiting the rate of spread of the virus in the community and protecting those most at risk of severe infections.

The virus was clearly contagious, spreading easily from one person to another and from one country to another. On 11 June 2009 the WHO raised its pandemic alert to phase 6, formally declaring a new influenza pandemic. By this time there had been nearly 30,000 confirmed cases reported in 74 countries.

In June 2009 it was also becoming apparent that most cases were having a relatively mild illness. However, there were more severe cases in young people than was typical for seasonal flu, and many of the more severely affected patients required high level hospital care, including sophisticated medical retrieval and intensive care services.

On 17 June 2009 Australia moved to the PROTECT phase, a new phase developed as the nature of the illness required greater flexibility in the level of response that had been previously planned for in the various federal, state and local government level pandemic influenza plans.

On 30 September the first doses of the pH1N1 vaccine were released. The vaccination program expanded with the release of a vaccine for those six months to less than ten years of age in December 2009. A summer vaccination campaign coinciding with the start of the 2010 school year followed to increase immunity to the pandemic virus.

Following this first intensive wave of the pandemic, NSW Health took the opportunity to review our response and determine which aspects were particularly successful and what could be implemented to strengthen future responses. Recommendations developed after consultation with NSW Health services and other key stakeholders (e.g. general practice and partner agencies) are presented in the NSW Health Key Recommendations on Pandemic (H1N1) 2009 Influenza.

On 10 August 2010 the WHO lowered its influenza pandemic alert level back to phase 3, signalling the end of the H1N1 pandemic.

The pandemic H1N1 (2009) virus has now become one of the seasonal influenza virus strains currently circulating in various parts of the world.

Would I be at risk during an influenza pandemic?

During an influenza pandemic, most people are at risk of becoming unwell. The effects of an influenza pandemic could impact upon our whole community and will vary from relatively minor to extremely severe depending on the severity of the illness caused in most people by the pandemic influenza virus.

Who is most at risk of getting a serious pandemic flu infection?

Every year people in certain high-risk groups are more likely to be severely impacted by seasonal flu than others. During an influenza pandemic we would expect that people in these same groups would also be at more risk of severe illness if they were infected, but there may also be other high-risk groups identified during a new influenza pandemic, depending on the nature of the new virus.

It is particularly important for people at increased risk of severe illness from influenza to seek medical attention early if they develop symptoms.

What can I do to prepare for an influenza pandemic?

You should prepare by considering the issues outlined in this guide and preparing a home emergency kit. Discuss how you and your household will cope during an influenza pandemic amongst yourselves and with other family and friends.

Most people have family and friends that they could turn to during a pandemic. Make sure you have an up‐to‐date list of phone numbers, e‐mail addresses and household addresses so that you can keep in touch.

For more information, please visit our pandemic page for families.

How can I stay healthy during an influenza pandemic?

The following measures will help you stay healthy:

  • Wash your hands with soap and water (or alcohol‐based hand cleanser) frequently.
  • Try to stay a metre away from sick people to reduce the spread of germs. Delay visits with people who are sick until they are better, if possible.
  • Eat a healthy diet and exercise regularly.
  • Get an annual influenza vaccination until a customised vaccine for the pandemic virus is available. It may not protect you from pandemic influenza, but it can prevent seasonal influenza.
  • If you are in a high‐risk group, such as being over 65 or having a chronic illness, talk to your GP about pneumococcal vaccination. It helps prevent pneumonia, which can follow an influenza infection.

What should I do if I get sick?

Special health arrangements will be communicated through the media. Follow the advice and directions of health officials.

For information on caring for young children, please visit our influenza page for managing influenza in young children.

Remember that health care may be delivered differently during a pandemic to ensure people are cared for at a time of high demand. Pregnant women or people who are in other high-risk groups should seek medical attention as soon as they develop symptoms of influenza.

What are the symptoms of pandemic influenza?

It is most likely that the main symptoms of pandemic influenza would be similar to seasonal influenza. These usually occur one to three days after infection, and may include sudden onset of:

  • fever
  • headache
  • muscle and joint pain
  • sore throat
  • cough
  • runny or stuffy nose
  • severe tiredness.

The symptoms of Pandemic (H1N1) 2009 were similar to the symptoms of seasonal influenza although more people also complained of vomiting and diarrhoea. It is also important to remember that some people can be infected with influenza yet show no symptoms or only very mild symptoms.

If your health deteriorates considerably or you are having trouble breathing, call: triple zero (000).

If a pandemic phone advice hotline is established, the number will be advertised widely.

How can I help stop the spread of pandemic influenza?

Some simple measures can help prevent the spread of pandemic influenza in the community and reduce the chances that more vulnerable people are exposed to the virus:

  • Wash your hands regularly, especially after touching used tissues.
  • Cover your mouth and nose with a tissue when you cough and sneeze
  • Stay at home and avoid close contact with other people if you are sick.
  • Keep sick children home from school, child care and other activities (e.g. sports training or playing with friends).
  • Avoid visiting people in hospital if you are sick. Your mild illness could be deadly to someone who is already unwell.
  • If you need to see a doctor, call ahead and advise the clinic you are coming. Your doctor may ask you to wear a face mask when you visit the clinic or direct you to a separate flu clinic for assessment.
  • Take your temperature at home with a thermometer ‐ this information can help your doctor assess your progress.
  • Paracetamol is good for aches and pains, and for reducing high temperatures. Adults can use aspirin-containing medications but these should not be used for children under 16 years because of the risk of Reye’s syndrome.

For more information, please visit our pandemic page for families.

What if schools and child-care centres close?

Children spread influenza easily. Schools and child‐care centres may close in an effort to control the spread of pandemic influenza. Children who have travelled to an affected area may also be asked not to attend school for a period of time after they return.

  • Plan now for children’s care as group care may not be available.
  • Have materials such as books, school work, craft and other recreational activities suitable for your children at home.
  • Help schools plan for pandemic influenza. Talk to your school principal, teachers and parent and citizen’s organisation.
  • Monitor the NSW Health website
  • Monitor the NSW Emergency website
  • Listen to local and national radio, watch news reports on television, and read your newspaper and other sources of printed and web‐based information.

How can I stay informed?

Monitor the NSW Health Emergency website (www.health.nsw.gov.au/emergency_preparedness) which will be updated with current health information.

Monitor the NSW Emergency website (www.emergency.nsw.gov.au) which will be also updated with other important information.

Listen to local and national radio, watch news reports on television, and read your newspaper and other sources of printed and web‐based information

What should I have in my home emergency kit?

Every family should have an emergency kit. The kit can be used in all types of emergencies, including an influenza pandemic, severe storms, floods and bushfires.

During a severe pandemic you and your family may have limited access to food, water and essential supplies. You should prepare a home emergency kit and ensure you have adequate supplies in your pantry to help you meet the needs of your family for up to 14 days.

Follow these links for information on preparing an Emergency Kit and for an Emerge​ncy Pantry list.

What is bird flu?

Bird Flu or avian influenza refers to a large group of different influenza A viruses that primarily affect birds. In fact, almost all influenza A viruses begin as avian influenza viruses but over time some of these viruses become adapted to other species, including humans.

The vast majority of avian influenza viruses do not infect humans. On rare occasions, these viruses can infect other species, including pigs, horses and humans. When avian influenza viruses adapt to be able to infect human there is a real risk that they may become a new pandemic influenza virus.

Currently, H5N1 bird flu (also called “highly pathogenic avian influenza (HPAI) influenza A/H5N1”) continues to circulate in bird species in a number of countries, with sporadic human cases reported. While this virus remains a major pandemic threat, other avian influenza A viruses (including H7 and H9 subtypes) have also led to human cases and could emerge as pandemic influenza threats if they adapt to be able to spread more efficiently from person to person.

For more information see:

What are H5N1 and H7N9 bird flu?

The H5N1 and H7N9 bird flu viruses are avian influenza viruses that occurs mainly in birds. They are among only a few of the avian influenza viruses that have crossed the species barrier to infect humans in recent times, and have caused large numbers of cases of severe disease and death in humans. Most cases are believed to have resulted from direct or close contact with infected poultry. These viruses do not infect humans easily, and if a person is infected, it is very rare for the virus to spread to another person.

However, because all influenza viruses have the ability to change there remains a real risk that these viruses could change into a form that is able to spread easily from person to person. If this adaptation occurs, it would no longer be a bird virus--it would be a human influenza virus. As there would be little or no immune protection against the new virus in the human population, an influenza pandemic (worldwide outbreak of disease) could begin.

Experts from around the world continue to monitor for potential changes in the these virus and changes in patterns of human infection. There is a need to plan and prepare for the possibility that the one of these viruses or another strain may begin to spread more easily and widely from person to person.

Current as at: Thursday 20 April 2023
Contact page owner: Communicable Diseases