Saturday, August 26, 2006

Bird flu vaccination can ‘worsen spread’

Bird flu vaccination can ‘worsen spread’
JAMES MORGAN

August 17 2006
Vaccinating poultry flocks against bird flu could make the spread of deadly strains such as H5N1 worse, Scottish scientists have found.

An Edinburgh University study concluded that, even though the available vaccines are effective on individual birds, the disease is likely to spread unless more than 95% of a flock has been protected.

The paper, published in the journal Nature, is the first to quantify how incomplete vaccination of flocks can contribute to the undetected spread of the disease.
In Vietnam, where 42 people have died of bird flu, mass vaccination of poultry has been successful in checking the spread of H5N1.

The Netherlands has already started unrolling a vaccination programme even though it has not yet been hit by H5N1.

But despite the Cellardyke swan, the UK authorities have ruled out a vaccination programme, on the grounds that it would disguise the spread of a much larger outbreak of H5N1.

"Silent spread" occurs because immunised birds can still catch and pass on avian flu, but without presenting any symptoms.

Therefore, as protection levels rise in a vaccinated flock, it becomes ever harder to detect the spread of avian flu, simply because fewer birds die. The result is increasing amounts of bird flu virus contaminating the birds' surroundings without farmers realising it.

The UK government's fears appear to be borne out by the new study, led by Dr Nick Savill, of the university's centre for infectious diseases.

Using computer modelling, Dr Savill found that, in practice, it is very hard to protect more than about of 90% of the birds in any given flock, and protection levels are usually much lower.

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