Occurs due to the emergence of a new flu virus which is markedly different from recently circulating strains.

• There were three pandemics last century. Occurs sporadically at any time of the year
• May affect around 25% of the population
• It could be a more serious infection for everyone
• People of every age may be at risk of serious illness
• A vaccine won't be available to start with – when it does become available the aim will be to immunise the whole population as rapidly as vaccine supplies come through
• Antiviral drugs are being stockpiled and will be used in the most effective way, according to how the disease develops

It could happen if:
(a) avian flu combines with ‘ordinary' flu or
(b) avian flu virus mutates and you get a completely new flu strain that can be
transmitted directly from one person to another.

It hasn't happened yet but if it did, it would be a big problem. Few – if any – people will have any immunity to the new virus. This allows it to spread widely, easily and to cause more serious illness.

How likely is a flu pandemic?

Experts predict another pandemic will occur but cannot say exactly when. Each pandemic is different and, until the virus starts circulating, it is impossible to predict its full effects.

How likely is it that pandemic flu will spread to the UK ?

The World Health Organization is on constant lookout for the first signs of a pandemic emerging so steps can be taken to slow its spread or contain it. If a pandemic flu strain emerges overseas, it will almost certainly reach the UK . Once it arrives, it is likely to spread throughout the country in a matter of weeks.

How will a flu pandemic differ from an outbreak of ‘ordinary' flu?

It is difficult to say just how serious a pandemic will be. It depends on how easily the particular virus spreads, which age groups are affected most, the severity of the symptoms it produces, and how many deaths it causes.

A flu pandemic may occur in two waves several months apart, with each wave lasting about two to three months.

It is likely that pandemic flu will affect far more people than ‘ordinary' flu. Around a quarter of the population may be affected by the end of the pandemic.

Who's at risk?

Once a pandemic of influenza starts, everybody will be at risk of catching it. Certain groups may be at greater risk than others: until the virus starts circulating we will not know for sure who the risk groups will be.

Is there a vaccine to protect against pandemic flu?

Medicines called antivirals can be used to treat influenza. They have been shown to be very helpful in the treatment of ‘ordinary' flu, and it is likely that they will also be effective in the treatment of pandemic flu, but their effectiveness won't be known until the pandemic virus is circulating. Antivirals do not stop the flu from developing but they do subdue the symptoms and reduce the time people are sick.

There is no vaccine ready to protect against pandemic flu. A vaccine to protect against pandemic flu cannot be made until the new virus has been identified. Before a pandemic starts it is difficult to predict what strain will cause it and even then, predictions may prove wrong. Also, the new virus may have changed enough for a preprepared vaccine to be ineffective.

‘Ordinary' flu vaccines will not protect against pandemic flu. But ‘ordinary' flu can be serious so it is very important that everyone who is due an ‘ordinary' flu jab has one.