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Pandemic flu

A pandemic is a worldwide outbreak of a disease that affects many people in different countries. An influenza, or flu, pandemic happens when a new strain of flu virus spreads easily and quickly across the world.

A flu virus is classed as a pandemic when:

  • a new strain of the flu virus develops
  • most people have no immunity against the virus – this means they may not be able to fight the infection
  • humans are affected and can pass the flu virus on to others
  • the virus spreads quickly and easily around the world

An epidemic is when more people are affected by a disease than usual. A pandemic is a worldwide epidemic.

Pandemic flu is similar to seasonal flu – the normal type of flu that tends to happen at around the same time every year – but the symptoms can be more severe. This is because few people will be able to fight off the infection easily, as it’s significantly different to previous forms of flu they have had.

More people are infected with the flu virus during a pandemic than are affected by seasonal flu. Seasonal flu tends to affect people in the winter, from October to May, but pandemic flu can happen at any time of the year.

In the 20th century, there have been several flu pandemics. The Spanish flu outbreak in 1918 and 1919 killed millions of people around the world. In 2009, there was a flu pandemic of the H1N1 flu virus (swine flu). It was the first pandemic for 40 years.

It's difficult to predict when a pandemic will happen, which virus might cause it or how many people might be affected. Pandemic flu can affect anyone, even the fit and healthy.

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  • Symptoms Symptoms of pandemic flu

    When you catch flu, it usually takes two to three days for your symptoms to show.

    Pandemic flu usually causes the same type of symptoms as seasonal flu. Flu viruses grow in the soft, warm surfaces of your nose, throat, sinuses, airways and lungs, so this is where you usually get the symptoms. The main symptoms are:

    • a fever (high temperature between 39ºC and 40ºC)
    • a blocked or runny nose
    • sneezing
    • headache
    • sore throat
    • cough
    • chills
    • aching muscles
    • feeling tired

    Symptoms usually last for about a week, but you may feel tired for a few weeks.

    If you think you have developed the symptoms of flu during a pandemic, you may be advised not to see your GP. However, if you're pregnant or have other health problems, such as diabetes or heart disease, you should contact your GP. Follow advice from the Department of Health about what to do in the event of a pandemic.

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  • Diagnosis Diagnosis of pandemic flu

    You won't usually need to see your GP if you have symptoms of flu during a flu pandemic. If your symptoms get worse or last longer than a week, or if you have a medical condition that may make flu worse, you should contact your GP for advice.

  • Treatment Treatment of pandemic flu


    If you have flu, you shouldn't visit your GP unless your symptoms get worse. There are a number of things you can do at home to help reduce your symptoms.

    • Drink enough fluids to prevent dehydration.
    • Stay at home and rest.
    • Take over-the-counter painkillers such as paracetamol, aspirin or ibuprofen.

    Always read the patient information leaflet that comes with your medicine and if you have any questions, ask your pharmacist for advice. If your child is under 16 years old, don’t give him or her aspirin or any medicines containing aspirin.


    Antiviral medicines are sometimes used to treat flu during a pandemic. However, until a flu pandemic starts, doctors can't be sure that antiviral medicines will work with that particular flu virus.

    Antiviral medicines can't stop you getting flu, but they may reduce your symptoms and the length of time you’re ill. Antiviral medicines work best if you take them within 48 hours of your symptoms starting.

    Antiviral medicines are usually only given to people who are at risk of severe illness if they catch flu, or to healthcare workers who care for those who are ill. Antibiotics won't help with flu symptoms, as they only work on bacterial infections. They might be used to help prevent complications from secondary bacterial infections, such as bacterial pneumonia.

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  • Causes Causes of pandemic flu

    The proteins that make up the flu virus are constantly changing (mutating). A flu pandemic can occur if there is a more dramatic change to the flu virus than is usually seen every year. This can happen if there is a mix of forms of flu from different species, such as birds or pigs, with a human form of flu. This is called an antigenic shift. This mix of different viruses can make a new, unique virus that no one will be immune to.

    Flu viruses are very infectious. Most people catch flu by breathing in air that has the virus in it. This usually happens when people with flu cough or sneeze, which spreads the virus in the air.

    You can also catch flu through direct contact with someone who has it, for example, by shaking hands or touching something they have touched. If you pick up the flu virus on your hands and then touch your nose or mouth, you may infect yourself. The flu virus can live on hard surfaces for up to 24 hours and on soft surfaces for about 20 minutes.

    If you have flu, you’re infectious and can spread the virus to other people. This can happen from the day when your symptoms start to five days afterwards. Children are infectious for longer.

  • Complications Complications of pandemic flu

    Healthy adults usually recover completely from seasonal flu in a few weeks. However, when pandemic flu develops, it’s difficult to know how severe the infection will be or how it may affect people. This is because the virus is a new strain. Some groups of people may be affected more than others. The complications of flu can include:

    • conditions that affect your lungs, for example, pneumonia and bronchitis
    • worsening of chest conditions, such as asthma
    • middle ear infections
    • inflamed sinuses (sinusitis)
    • myositis – sore leg muscles
    • encephalitis – inflammation of the brain
    • febrile convulsions – these are fits or seizures caused by a high temperature and seen mainly in children
  • Prevention Prevention of pandemic flu

    Pandemic flu can't be prevented, but there are a number of things you can do to reduce the spread of the virus and your risk of catching it. Some of the main ones are listed below.

    • Cover your nose and mouth with a tissue when you cough or sneeze.
    • Put used tissues into a bin as soon as you can.
    • Wash your hands with soap and water frequently, especially after you sneeze or cough.
    • As well as washing your hands frequently, use an alcohol-based liquid or gel hand sanitiser, for example, after coughing or handling used tissues.
    • Clean hard surfaces like door handles and desks frequently.
    • Stay away from large crowds of people.
    • Don’t travel unless it's necessary.
    • If you're in a risk group, have the currently recommended flu vaccine once it becomes available.

    Make sure your children follow these guidelines too.

  • Vaccines Vaccines for pandemic flu

    Vaccines are always available for seasonal forms of flu. However, when a pandemic begins, it usually takes several months to make a new vaccine. In a pandemic flu period, once the vaccine becomes available, you will be advised whether you're in a risk group that should receive the vaccine. The risk groups identified for the vaccine are subject to change. They may include people aged over 65, and those with underlying diseases such as chronic lung disease, heart disease, diabetes and cancer.

  • FAQs FAQs

    How will I get antivirals if a flu pandemic begins?


    In the event of a flu pandemic, the Department of Health will set up a National Pandemic Flu Line, which you can contact for more information.


    Antivirals can currently only be prescribed by a doctor. However, the UK Government is working on legislation and guidelines to ensure that during a pandemic, people who become ill will be able to have easier access to antivirals. It's planned that people will be able to phone the National Pandemic Flu Line to be assessed and get authorisation for treatment over the phone. Antivirals may reduce the length of time symptoms last and how severe they are. It's not yet known whether they will prevent any serious complications of pandemic flu, such as pneumonia.

    Will a seasonal flu vaccine protect me against an outbreak of pandemic flu?


    No, a seasonal flu vaccine will not protect you against a new strain of pandemic flu.


    Vaccines are usually made from a killed or weakened form of a virus. Scientists must have the exact strain of virus before they can create the vaccine. Seasonal flu only changes a little from year to year, which means that the virus can usually be predicted in advance. This means that vaccines can be prepared and given in advance. Every year, people who are most at risk from seasonal flu are offered a vaccine against the virus.

    However, pandemic flu is unpredictable and can happen at any time. The flu viruses that cause pandemic flu are new strains – and it's impossible to predict in advance whether a new strain of flu will cause a pandemic. This means it's impossible to produce a vaccine until a pandemic has already begun and scientists are able to identify the virus responsible.

    Once a vaccine for a pandemic strain has been produced, it may be included as part of the next seasonal flu vaccine. For example, this happened with the vaccine for the H1N1 swine flu strain in the 2010/2011 flu season. By this point, the virus is no longer classed as a pandemic as enough people in the population have immunity.

    Should I wear a face mask during a pandemic flu outbreak?


    If you have flu, wearing a face mask may help to prevent you from passing it on to someone else. If you don't have flu, wearing a face mask is unlikely to prevent you catching it.


    If there is a flu pandemic, you may think about wearing a face mask to protect yourself from the virus, or to prevent your infection spreading to someone else. Most people catch flu by breathing in air that has the virus in it. This usually happens when people with flu cough or sneeze, which spreads the virus through the air.

    If you have flu, wearing a face mask may help to stop you from passing it on to someone else. However, wearing a face mask if you don’t have the flu is unlikely to prevent you catching it.

    I'm travelling to Asia - should I be worried about avian flu?


    Some countries in Asia have had outbreaks of avian (bird) flu. However, the risk of catching the virus is low.


    Avian flu usually only affects birds. In some birds, it only causes a few problems, but in poultry, for example chickens, it can spread quickly and kill large numbers of birds. Sometimes, the avian flu virus can spread to people, usually those who work in close contact with poultry. However, the virus usually doesn’t spread from person to person.

    In recent years the avian flu virus has killed about 300 people. Many of these deaths were in Asia in people who had close contact with birds. If you're travelling to a country where there has been an outbreak of avian flu, the Health Protection Agency recommends you should:

    • not visit places where live poultry are raised or kept. For example, poultry farms or bird markets, where you could be in contact with ill or dead birds
    • make sure that chicken, egg or duck dishes are cooked thoroughly before you eat them
    • avoid touching surfaces that could be contaminated with animal faeces
    • wash your hands with soap and water frequently

    The risk of catching avian flu is low, but if you develop the symptoms of flu during or after a trip to Asia, contact your GP.

  • Resources Resources

    Further information


    • UK influenza pandemic preparedness strategy 2011. Department of Health, 2011.
    • Frequently asked questions on influenza. Health Protection Agency., published 13 November 2013
    • Influenza virus infections in humans. World Health Organization., published July 2013
    • Pandemic influenza preparedness and response. World Health Organization., published 2009
    • H1N1 (2009) pandemic archive. Health Protection Agency., published 11 March 2011
    • Influenza – seasonal. NICE Clinical Knowledge Summaries., published October 2013
    • The use of facemasks and respirators in an influenza pandemic. Department of Health, 2011.
    • Seasonal influenza: factsheet for health professionals. European Centre for Disease Prevention and Control., accessed 2 December 2013
    • Avian influenza and pandemic flu. Health Protection Agency., published 13 August 2008
    • Travel advice. Health Protection Agency., published 31 December 2008
    • Influenza. BMJ Best Practice., published 21 October 2011
    • Influenza. PatientPlus., published 11 October 2013
    • Q & A on seasonal influenza. European Centre for Disease Prevention and Control., accessed 3 March 2014
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