IT may sound like a Doomsday scenario: a constantly changing virus which has the ability to strike without warning and kill millions.
But that is the prospect medical experts have to consider when dealing with one of winter's most common illnesses: flu.
With constant fears that a major pandemic of the illness could be on the horizon, scientists in Edinburgh are trying to decode the basic genetic structure of the virus to produce better anti-viral drugs and vaccines. The endeavour comes as figures emerged showing that fatal flu cases in Scotland this year are more than double those of last year.
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The Scottish flu researchers believe they have now made a breakthrough which could enable production of the flu jab to be speeded up to meet the huge demand which would be required in the event of a major outbreak of the illness.
The production of the vaccine relies on successfully growing specific strains of the virus in hens' eggs, based on predictions of which types are thought to be the most likely candidates to be circulating in a particular year.
However, Professor Paul Digard, chair of virology at The Roslin Institute at Edinburgh University, said problems could be experienced should demand suddenly increase, as it did during the global swine flu pandemic of 2009 which killed half a million people across the world.
He said: "Some vaccine strains don't grow as well as you would like. Our research has recently thrown up some ideas on how we can make the vaccine strain grow better.
"If that works, we could double or treble the amount of vaccine. If you can get 100 million doses instead of 50 million doses of vaccine, that is actually quite a big deal."
The research is based on understanding the "jigsaw" of how the different proteins on the surface of the virus – the components which can mutate rapidly, creating new strains – interact with each other.
Digard said: "When you are making a vaccine strain of (flu) virus, you start by taking bits from different viruses and gelling them together.
"So, if you understand how they are meant to fit together, then you can change it so the vaccine virus is going to be a better-growing strain."
The initial research has been carried out and now Digard's team is seeking funding to develop the idea further: "If there is an outbreak next year, it is too soon for what we are doing now to have any effect. But if it is 10 years down the line it is a realistic prospect – perhaps even in five years."
Latest figures show seasonal flu has so far claimed the lives of 11 people in Scotland this winter – two last week – and resulted in 53 people being admitted to intensive care. Last year there were five deaths while in 2010-11, 63 people died from the virus in the wake of swine flu. Across the UK, a total of 36 confirmed influenza deaths have been reported so far this year.
Dr Jim McMenamin, consultant epidemiologist with Health Protection Scotland, pointed out that figures in England may not be complete as not all NHS Trusts had returned figures.
However, he said there were more cases in Scotland this winter compared to last year, which had seen the "latest and lowest flu activity for decades".
"Most cases observed in intensive care have been this side of the new year, because until the very tail-end of 2012 we had only seen about 11 admissions," he added. "Since then we have seen most of the admissions and sadly the majority of the deaths reported."
While flu is usually associated with the winter months – one factor for its virulence is the cold weather helping the virus survive longer. McMenamin pointed out that the flu season could stretch until the third week of May.
He added: "It's never too late to be vaccinated. There is continuing influenza transmission in Scotland and, for those people most at risk of influenza, the best protection for themselves and their family is to take up the offer of a free flu vaccine."
According to latest data, about 75.9% of those aged over 65 have had the flu jab. However, only about 55% of those under that age in another "at-risk" category – such as people with a chronic health condition – have had the vaccine, and the figure for pregnant women with no other risk factors falls to 49%.
Julie Smart, 35, a beauty therapist from Dunfermline, usually has the vaccine as she has asthma, but this year made certain she had the jab as she is due to give birth in March.
She said: "I knew quite a bit about it before, but the midwife went through how important it is to have it done for you and the wee one. If I didn't have asthma, it is something I would have done anyway."
While seasonal flu is an annual threat, the fear is that the flu virus could mutate rapidly and trigger a pandemic, such as the Spanish flu of 1918 which killed up to 50 million people around the world.
Over the past five years, the Scottish Government has spent more than £90 million on preparing for such a scenario, with most spent during 2009-10 to 2010-11 to tackle swine flu. This virus – H1N1 – first appeared in Mexico in 2009 and spread rapidly around the world, but turned out to be less deadly than initially feared.
However, John Oxford, professor of virology at Queen Mary, University of London, points out it still killed nearly half a million people around the world and predicts that another pandemic could strike in as little as five years time.
Oxford says he believes the two most likely candidates to trigger a new pandemic are avian flu (H5N1) and Asian flu (H2N2), which caused a global outbreak between 1957 and 1968.
He is one of a number of researchers around the world racing to produce a "universal" flu jab, which would work against various strains and could be stockpiled. Rather than being based on the constantly changing proteins on the surface of the virus, it would use the more stable proteins inside. A vaccine created from these stable proteins will be more effective as there is less chance of it being rendered ineffective because of the flu mutating. However, these "internal" proteins are less likely to produce the immune response needed to be effective.
"But sometimes that luck runs out, the judgment is wrong and there is the wrong component in the vaccine. If you had the so-called universal flu vaccine, one with a broader protective effect, that sort of action would be very useful indeed."
In the meantime, the battle to stay one step ahead of flu goes on. Edinburgh University virologist Digard said it is one of the most difficult infectious diseases facing modern medicine: "It is a dangerous virus, because it is unpredictable. None of us expect to die from the likes of polio and our children shouldn't die from polio, as long as we get them vaccinated. But you can't say the same thing about flu necessarily."