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Last Updated: Wednesday, 22 February 2006, 10:17 GMT
Bird flu vaccine no 'silver bullet'
By Rebecca Morelle
BBC News science reporter

Hen (AFP/Getty)
Vaccination could stop the train of transmission
Vaccinating the UK's poultry is not currently needed as a precautionary measure against bird flu, many leading animal health experts say.

Only if an outbreak occurs and other preventative strategies fail should the government consider the option, they told the BBC News website.

Current bird flu vaccines do not offer complete protection from infection and could, in some circumstances, "hide" the virus in affected flocks, they believe.

A government spokesperson said that, at present, vaccines were "not a path that they want to go down".

"Vaccination offers potential benefits but currently available vaccines are too limited to provide a general solution," he added.

Increased vigilance

The UK has not yet been struck by the virulent form of bird flu, H5N1; but ministers admit the chances of it arriving are increasing and have called for raised vigilance.

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Were an outbreak to occur, emergency measures would be enacted. Infected flocks would be culled and bio-secure zones would be set up around affected farms - but vaccination of infection-free birds is not planned at present.

Stephen Lister, a poultry veterinarian, told the BBC News website that, in his view, this tactic should be adequate.

"As an island country, I think that vigilance, surveillance and the stamping out of any case that appears very quickly is the best way to approach it," he explained.

It is a tool to help us control the situation, but not a 'silver bullet' to wipe the disease out
Stephen Lister, poultry vet
"I can't see enough to warrant us vaccinating all of our susceptible birds, because I don't think the risk is high enough at the moment."

However, scientists agreed that vaccination should not be ruled out as it could be a useful measure for controlling the disease if it spreads in the UK.

"The obvious benefit is that you can stop the virus in its tracks," said Dr Wendy Barclay, a virologist at the University of Reading.

"And the fewer number of chickens infected, the less the exposure of people; and from an economic point of view, you lose fewer birds. It's all about breaking the train of transmission."

Animal welfare

Dr Bob McCracken, president of the British Veterinary Association, also said vaccination should be considered for free range birds.

"We don't necessarily need to be thinking about vaccinating the entire poultry population in the UK," he said, "but we should consider vaccinating those free range birds that have been brought indoors.

If you vaccinate you will protect from disease, but you will not necessarily give complete protection from infection
John McCauley, Institute of Animal Health
"The time will come when we have three options. We can turn these birds out and risk infection. Keep them in but recognise this would be very detrimental to their health and welfare; or we could vaccinate them."

But the scientists stressed that while vaccinating against bird flu can be advantageous, there are problems associated with it.

"If you vaccinate you will protect from disease, but you will not necessarily give complete protection from infection," explained Dr John McCauley, at the Institute of Animal Health.

HOW VACCINES WORK
Vaccines for poultry in China  (Getty Images)
All vaccines trigger immune response to ward off disease
Attenuated vaccine: contains weaker version of live virus
Inactivated: contains virus inactivated by heat, chemicals
Subunit: introduces gene to make disease antigens
Vaccines slow to stockpile; grown in chicken eggs
Flock needs simultaneous inoculation to be effective
Some vaccines require two doses for full immunity
"So whilst the birds are infected with the highly pathogenic flu virus which would normally kill them, they are not dying."

Dr Barclay added that this could be dangerous for human health, increase the risk of passing the virus from poultry back into wild birds, and be worrying in terms of virus mutations.

"The Chinese have made a vaccine based on reverse genetics made with H5N1 antigens, and they have been using it," she said.

"There has been a lot of criticism of what they have done, because they have protected their chickens against death from this virus but the chickens still get infected; and then you get drift - the virus mutates in response to the antibodies - and now we have a situation where we have five or six 'flavours' of H5N1 out there."

Vaccine stockpile

One method of protecting against this would be to introduce "sentinel birds" into vaccinated flocks. These purposefully unvaccinated birds would indicate if the flock has become infected by the H5N1 virus.

Dr John McCauley also cautioned that once vaccination was stopped, infection could arise again.

"When you vaccinate you will reduce the intensity of the infection and disease, but when you stop vaccinating the disease will come back again, because you haven't eliminated it," he said.

Stephen Lister concurred: "It is a tool to help us control the situation, but is not a 'silver bullet' to wipe the disease out."

Professor Neil Ferguson, of Imperial College London, said a long-term strategy for bird flu in the UK was needed.

"I think it is maybe a little bit premature for us to be vaccinating right now in this country, but we need to at least be stockpiling vaccine," he said.

"I think we need to be thinking what we will be doing in 12-48 months' time in terms of having a sustainable policy which minimises the chance of the infection getting into domestic poultry."




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