Healthy living * Travel health

What is swine flu?

background information on swine flu Background on the virus

Swine flu is the name given to a completely new influenza virus, first detected in humans in Mexico City in April 2009. It is one of many influenza type A virus strains which cause outbreaks of flu during the winter months each year in the UK.

It is called swine flu because scientists initially thought the virus came from pigs. Type A influenza viruses can infect humans and animals, including pigs and birds. If a host animal becomes infected with both human and animal/bird (avian) flu strains, it can give rise to a completely new strain of influenza.

It is now known that the new H1N1 virus has genes from strains of pig, human and bird flu viruses. The original source of the new H1N1 virus is still unknown. Last week the WHO requested the new strain be called H1N1/09 virus, to avoid stigmatizing Mexico or the pork industry, but the swine flu name has stuck.

How does H1N1/09 flu compare to ordinary seasonal flu?

In the UK, seasonal flu occurs every year during the winter months of October to April. This is caused mostly by influenza type A viruses already circulating in the population.

Many people are either immune or have partial immunity to these flu viruses, and typically only 10% to 15% of the population become infected. Although anyone can catch seasonal flu, people with chronic diseases, the very young and old people are most likely to suffer from complications such as pneumonia, which can be fatal. Each year, an estimated 12,000 people in the UK die from flu-related illness.

Minor changes to seasonal flu viruses are common, resulting in new strains. Fortunately, scientists can often predict these, and new vaccines are prepared in advance for winter.

Because the H1N1/09 influenza virus varies so much from seasonal flu virus strains, people have little, if any immunity to it. This is why it has spread so rapidly, causing a pandemic (a global outbreak of disease). According to the WHO, H1N1/09 flu has spread with "unprecedented speed."

It also means that more people will be infected. In a worst case scenario, the UK government is prepared for up to 30% of the population falling ill with flu. They also estimate that 65,000 people could die from flu related illness, based on a mortality rate of 0.35%.

Grim as these figures may seem, so far the H1N1/09 virus causes only mild disease in most people, unlike the pandemic flu viruses of 1918, 1957 and 1968. Most people recover without being treated. However, in a minority of cases the virus causes severe disease.

Unlike seasonal flu, the under-5s and the 5 to 14 age groups are predominately infected. The UK government has stated that up to 50% of children may be infected when the pandemic peaks in the UK. People at higher risk of disease complications are the same as for seasonal flu, but also include pregnant women and, perhaps, people who are obese.

As it is not possible to predict when a pandemic is going to occur, there is no vaccine against the H1N1/09 influenza virus. However, one is in development and 132,000 doses have been ordered, enough for the entire UK population.

TheFamilyGP guide to swine flu

Action plan

For tips on how to be prepared for swine flu, and important contact numbers and sources of information.

Swine flu background

Risk groups

Progress of the disease

This article was published on Fri 17 July 2009

Image © CDC C. S. Goldsmith and A. Balish

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