Influenza (flu)

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Influenza, or ‘the flu’, is a very common and highly infectious disease caused by a virus. It leads to hundreds of thousands of GP visits and more than ten thousand hospital stays each year.

It can be very dangerous and cause serious complications, sometimes even death. This is more common for some people in at-risk groups, for example, babies under six months, older people, and those with certain long-term medical conditions.

In the UK it is estimated that an average of 600 people a year die from complications of flu. In some years it is estimated that this can rise to over 10,000 deaths (see for example this UK study from 2013 , which estimated over 13,000 deaths resulting from flu in 2008-09).

The flu virus constantly changes, so different strains circulate yearly.  The World Health Organization monitors the virus throughout the world and advises which 3 or 4 strains should be covered by an annual flu vaccine. Vaccination from previous years is not likely to protect people against current strains of flu.

Flu epidemics can kill thousands or even millions of people. The 1918 flu epidemic is estimated to have affected half the world's population and 40-50 million people died worldwide. The World Health Organization (WHO) estimates that between 250,000 and 500,000 people around the world die from the flu every year.

In the UK (and in the rest of the northern hemisphere) the annual flu season runs from about October to March or April. Most cases of flu occur between December and February.


Compared to a common cold, the symptoms of flu are generally more severe and last longer. The fever also develops more suddenly and is usually higher than with a cold.

The most common symptoms of flu are:

  • a sudden high temperature of 38°C or above
  • severe headache
  • general aches, pains, tiredness, and weakness
  • shivering and chills
  • aching muscles, pain in limbs or joints
  • sore throat
  • a runny or blocked nose, and sneezing
  • a dry, chesty cough
  • difficulty sleeping

However, sometimes flu can look like other types of illness, especially in children, so it can be hard to recognise:

  • diarrhoea
  • pain in the stomach
  • loss of appetite
  • nausea and vomiting

It is the complications of flu that are dangerous. The most common complication is a bacterial chest infection, which can develop into pneumonia. Other complications include:

  • Middle ear infection
  • Septic shock - a severe and life threatening infection of the whole body
  • Encephalitis - inflammation of the brain
  • Meningitis - inflammation of the covering of the brain.

See the Meningitis Research Foundation  website for more detailed information on the signs and symptoms of meningitis.

Serious complications can affect anyone, even healthy people. They are more common in babies under six months, older people, those with certain long-term medical conditions, and pregnant women. During pregnancy, the baby may be affected, causing premature birth, low birth weight, or even death.


Flu is passed on by coughing and sneezing. A person can be infected either by breathing in droplets of the virus in the air or by touching surfaces on which they have landed.

The virus can live for around 24 hours on things like computer keyboards, handrails, and door handles etc.

Around 1 in 3 people infected by the flu virus will not show any symptoms, but they can still pass it on to others. Those who do show symptoms will be infectious for a day before symptoms develop and for a total period of about a week.

Children may remain infectious for up to two weeks. They are more vulnerable to flu than adults and more likely to pass on the infection.


In the UK, the flu vaccine is available each year from late September or early October onwards. Eligible people are recommended to get their flu vaccine in the autumn or early winter before outbreaks begin.

It takes up to two weeks after vaccination for protection to develop. In the 2020-21 season, more than 19 million adults and children were vaccinated against flu.

For the 2023-24 UK flu season, three different types of inactivated flu vaccines are available, with an extra version that has an added ‘adjuvant’ to make the vaccine more effective in older people.

There is also a live nasal flu vaccine available which is available for children over 2 years of age. All the flu vaccines used in the UK are currently ‘quadrivalent’ and protect against the four strains of the flu that are likely to spread.

Good personal hygiene and cleaning of surfaces can also help to prevent the spread of the flu.

Vaccinations are now set to start on 11 September 2023 in England with adult care home residents and those most at risk to receive vaccines first.

Visit the flu vaccine page for more information about the various flu vaccines available.


Page last updated Monday, November 06, 2023