How would the NHS cope with a quarter of the population ill or dying from flu? Mark Gould reports on the Department of Health's contingency plan for the next flu pandemic, which experts believe is on its way shortly

'We are like Volcanologists. We are sitting by the volcano expecting it to erupt. We don't know when, but our fingers are getting twitchy,' says virologist Professor John Oxford.

He is talking about an influenza pandemic - a worldwide epidemic - which he believes is a very real possibility in the next few years.

The Bart's and Royal London Hospital medical school microbiology professor likes to employ destructive natural imagery - volcanoes, hurricanes - to describe his fascination with the ever-mutating influenza virus.

Speaking a fortnight ago at the launch of this year's vaccination campaign, which hopes to deliver more than 8 million£5 jabs before the flu season proper kicks off in late December, Professor Oxford set out the problem for the NHS.

'It's a very remarkable virus. You have to reformulate the vaccine every year because the virus changes its composition every year. It's a number of years since the last pandemic, and it's not just me that is worried. The Department of Health has a pandemic contingency plan and so does the World Health Organisation.'

The last pandemic was in 1968, but in 1989 an epidemic in the UK killed 26,000 people. The Spanish influenza pandemic of the winter of 1918 killed 25 million - more than the death toll of the First World War.

Intervals between pandemics have varied from 11 to 42 years but offer no clue to the arrival of the next one.

Sometimes there are 'benign' pandemics, in which a new strain sweeps the world without massive fatalities.

The last one was in 1977 and primarily affected those born after the 1950s.

Given that it has been 30 years since the last big killer, the DoH has produced contingency plans for organising health and other services to deal with mass illness and death which could hit a quarter of the population, closing businesses, schools and vital services.

The DoH plan concedes that it will be a race against time to isolate the new flu virus and develop, manufacture and distribute a vaccine. Even then it cannot be sure that enough vaccine will be available. Included in the plan is a worrying set of questions and answers.

It reads: 'The UK will discuss its needs with manufacturers, but it is unlikely enough vaccine will be able to be produced for everyone to be immunised.' Does this mean vaccine will be rationed?

'Almost certainly vaccine will be given to certain groups first. These will be extended as more vaccine becomes available.'

In the event of a pandemic, all health services, emergency services, armed forces, utilities and undertakers would be vaccinated first, to ensure essential services were not disrupted.

As it is, all health staff are being advised to be vaccinated this winter due to the added pressures of the millennium holiday. During the 1968 pandemic, hospitals in Liverpool reported that a third of nursing staff were off sick.

The DoH plan is dated March 1997 and is already out of date: it refers throughout to an extinct species - fundholding GPs.

It aims to reduce death and severe illness, 'cope with large numbers of people ill at home and in hospital and the dying', and ensure that essential services are maintained during the six to eight-week period when the virus will be at its peak.

The government will try to keep a lid on panic by 'providing timely, authoritative and up-to-date information to professionals, the public and the media at all stages'.

Deaths occur because massive novel viral load knocks out people's immune systems.

Complications caused by secondary pneumonia-type infections are also a big killer.

Trust chief executives will be asked to co-ordinate mortuary arrangements 'in the event of a large number of deaths' and draw up plans to recruit retired doctors and nurses to replace sick staff.

This year's UK vaccine is a cocktail to protect against four closely related flu strains: Sydney A (H3N2), Beijing A (H1N1), a Beijing B-like virus and a Shangdong B-like virus. The A-class strains are the big killers and their H and N components are the molecules that can mutate and cause problems.

One of the strains that has been preoccupying Professor Oxford and the world's other influenza chasers is A (H5N1) - 'avian flu' - the mutant virus that led to the slaughter of every chicken in Hong Kong in 1997, and hospitalised 18 people, with six fatalities.

The WHO says A (H5N1) is as aggressive as the Spanish influenza that killed millions of young adults after the First World War.

The last confirmed death from complications sparked by A (H5N1) virus was in January last year. The Hong Kong Standard reported that a 25year-old Filipino domestic helper died from pneumonia complications. Her family suspected that she had contracted the disease when she prepared chicken soup for her employer.

To date there is no commercially available vaccine for A (H5N1).

And then there is a pig-related influenza that has been detected in the US.

Animal-to-human transmission of a new flu strain is all too possible, but the pandemic, human-to-human transfer is still waiting to happen.

A senior US WHO virologist, Dr Robert Webster, says: 'We can't relax yet. It may take months or years before we can.

'This virus could still be ticking over in the human population of Hong Kong .'

In the event of a new mutation being detected by Professor Oxford and his colleagues in the WHO's 120 influenza surveillance centres, the WHO will inform the DoH and the Public Health Laboratory Service.

The government will then have a short time to act. International air travel means that an aggressive virus would take just a few hours to reach the UK, rather than the six months taken by the 1918 killer.

The health secretary will convene the influenza advisory committee, chaired by a minister, chief medical officer or senior DoH official, and comprised of senior managers and clinical representatives, a mathematical modeller, and representatives from trade and professional groups, the royal colleges and the media.

The National Institute for Medical Research in Mill Hill, north London, will obtain viral samples that may be suitable to put into mass vaccine production by drug companies.

The Royal College of General Practitioners will put its 'spotter' GP practices on the alert for the new strain, and health authorities, trusts and 'fundholders' will be asked to prepare action plans, identify local coordinators and build up banks of staff to deliver vaccines and provide care for the sick and dying.

Schools could be closed to help prevent the spread of the virus, but that decision would rest with the prime minister.

The UK contingency document is something of a cliche of British restraint.

But the WHO's planner takes a noholds-barred approach: 'We must recognise that no pandemic plan prepared in advance will be 100 per cent relevant or best for whatever situation nature eventually creates.

'Hence emphasising that the process and issues for responding to a possible or actual pandemic may be more important than specific details, which may prove inapplicable to a new situation.'

Professor Oxford has a Blue Peter - style model of the virus which he brings out to educate journalists.

As his hands move across its surface, he says: 'It's fascinating because the virus changes year on year. It's like a moving target or a chameleon.'