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One of the main policy options for limiting the threat of bird flu could encourage the spread of the virus if it is only implemented partially, researchers have warned.

In a paper published tomorrow (17 August) in Nature, they say that vaccinating poultry flocks against H5N1 will only be effective if the vaccine protects at least 95 per cent of birds.

A less effective vaccination programme would encourage the 'silent spread' of the virus between flocks, warn the researchers, led by Nicholas Savill of the University of Edinburgh in Scotland.

Concern that vaccinated birds could pass the virus onto others has led some countries, including Thailand, to ban vaccination.

Savill's team used mathematical models to simulate the spread of H5N1 through caged flocks of 10,000 birds that had been contaminated with a small amount of infected faeces.

They conclude that 90 per cent of birds need to be protected by a vaccine to reduce the risk of an outbreak by half, but say this could mean some outbreaks escape detection and spread to other flocks.

The researchers say that at least 95 per cent of birds need to be protected to prevent the virus spreading silently.

In practice, it is difficult to protect more than 90 per cent of a flock; protection levels achieved by a vaccine are usually much lower than this.

Savill told SciDev.Net that using unvaccinated 'sentinel' birds would allow farmers and veterinarians to rapidly detect bird flu if the sentinels fell ill, thereby reducing the risk of the virus spreading.

He said that small household flocks and commercial flocks face the same problem.

But vaccination against H5N1 is expensive and hard to implement in rural areas of developing countries, says Robyn Alders of the International Rural Poultry Centre.

Alders says vaccination should be considered only when an outbreak has been confirmed, first line defences such as culling and decontamination have failed, and the disease is spreading. 

She said it is important to remember that in villages, there will always be an increasing number of sentinel birds between vaccination campaigns as chicks are constantly hatching.

Savill's study says that vaccination should be part of a wider control strategy that includes surveillance, education, restrictions on the movement of poultry, and efforts to diagnose and eliminate infected birds.

Link to full paper in Nature

Reference: Nature 442, 757 (2006)

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