Back to contents

Influenza ('flu')

Greta Hughson, Selina Corkery
Published: 17 October 2013

Influenza, usually known as the flu, is a very common viral illness that affects the respiratory tract.

There are different types of flu viruses, and different strains within those types, with strains changing and new strains emerging over time. A new strain can cause more serious illness because people will not have been exposed to it before and therefore won’t have built up any resistance to it.

For example, H1N1 is a relatively recent strain of the influenza virus, known as swine flu. It was first identified in April 2009, and declared a global pandemic in 2009/10. The epidemic was not as serious as feared in the UK, with most cases being relatively mild. However, some people became more seriously ill and some died, including previously healthy people. It spread quickly because of a lack of resistance to it. H1N1 is no longer considered a pandemic, and there have been relatively few cases of this strain of the flu virus during the UK's recent winter flu seasons.

How is flu spread?

Flu is very contagious and spreads from one person to another in small droplets of saliva when someone with flu coughs or sneezes – the same way colds are spread. The virus can live on a hard surface for 24 hours and on a soft one for about 20 minutes. People can be infected through the droplets left by a person with flu on objects such as phones, keyboards and door handles.

The incubation period is usually between two and five days but can be up to seven days in children. People are most infectious soon after they develop symptoms and are not considered infectious anymore once their symptoms have disappeared.

You can get flu at any time of the year, but it is more common during the winter. Because of this it is sometimes referred to as ‘seasonal’ flu. In the UK, a flu vaccination programme is provided each year that runs during the autumn (see below for more information). 

What are the symptoms of flu?

People usually develop a fever or high temperature (over 38°C or 100.4°F), often quite suddenly, as well as any of these symptoms: tiredness; headache; a runny or blocked nose; sneezing; sore throat; chills; a dry, chesty cough; loss of appetite; aching muscles; diarrhoea.

These symptoms are usually at their worst two or three days after infection, and start to improve after about seven days. It may take you longer to feel completely better. The flu is different to the common cold and the symptoms tend to be more severe and to last longer.

Most people recover without medical treatment, but flu can, in some cases, lead to more serious illness. People at higher risk of this include pregnant women, people aged 65 and over, people with long-term medical conditions (such as respiratory diseases like asthma or tuberculosis [TB], diabetes or chronic kidney disease), and people with a weakened immune system.

Having a low CD4 cell count (under 200) can increase the risk of complications. Having a condition such as asthma or TB as well can increase the risk further, especially as complications are most often bacterial chest infections such as bronchitis or pneumonia.

Treating flu

Generally, you should stay at home, rest, drink plenty of fluids, keep warm, and use over-the-counter painkillers to relieve symptoms.

Flu can be treated with the antiviral medicines oseltamavir (Tamiflu) and zanamivir (Relenza). The drugs must be taken at or near the start of the illness to be effective. They are not a cure, but can relieve symptoms, shorten the length of time you are ill and reduce the risk of complications. Antibiotics will not work, as flu is not bacterial.

You should contact your GP if you have symptoms of flu and you have a serious existing illness (HIV is not considered a ‘serious illness’ for people with flu if you have a CD4 count of over 200 and you are otherwise in good health) or if you are pregnant. Similarly, contact your GP if a child under twelve months shows symptoms of flu.

You should contact your GP or go to A&E if you have flu and:

  • your condition suddenly gets much worse.
  • after seven days (five for a child) your condition is still getting worse or fails to respond to antiviral drugs.

Preventing flu

Good hygiene is the most effective way of stopping the transmission of viruses such as flu viruses. Washing your hands regularly with soap and water and cleaning surfaces regularly will help prevent infection. For people with flu, sneezing and coughing into a tissue, throwing the tissue away and then washing your hands, will also help prevent onward transmission.

Flu can be prevented (as well as treated) with the antiviral medicines oseltamavir (Tamiflu) and zanamivir (Relenza). In some situations, people with HIV may be prescribed these as a preventive measure.

If you do have a low CD4 count or have an AIDS-defining illness, it would be a good idea to check with your HIV clinic whether you could be taking any other action to reduce your risk of getting flu, but the most useful action you can take is to have an annual flu vaccination.

The flu vaccine

Vaccines against flu viruses have been available for many years. Because flu viruses are so susceptible to change, the make-up of the vaccine is reviewed each year to include the most prevalent strains.

It is recommended that people in high-risk groups, including people with HIV, be vaccinated. The vaccine only provides protection for about a year, so it is important you are revaccinated each year.

In the UK, people with HIV are entitled to receive the vaccination free of charge, as are people aged 65 and over and all pregnant women. The vaccine is available at GP surgeries. You will need to be registered with a GP and to have told them of your HIV status to be eligible. If you have a low CD4 cell count, anyone you live with may also be eligible for free vaccination. Ask your GP about this. You can also talk to your GP about having a vaccination against pneumococcal disease.

The seasonal flu vaccine doesn’t have ‘live’ virus in it, so it is safe for use in people with HIV. The vaccine is effective in people with HIV, although there is some evidence that people with low CD4 counts are less likely to be protected by it (however, it may reduce the risk of complications).

Your GP should take into account other drugs you are taking that may interact with the flu vaccine. You can check for any reported interactions between the flu vaccine and anti-HIV drugs at www.hiv-druginteractions.org.

There is more information on flu vaccination on the NHS Choices website (www.nhs.uk).

Contact NAM to find out more about the scientific research and information used to produce this factsheet.

This content was checked for accuracy at the time it was written. It may have been superseded by more recent developments. NAM recommends checking whether this is the most current information when making decisions that may affect your health.