The creation of the deadly new variant was only possible because the vaccines contained live viruses, even though they were weakened forms, said Joanne Devlin, lead author of the paper published in the journal Science.

Devlin and her team discovered how closely related the two new strains were with viruses in the vaccines after analyzing their genes.

"What we found was the field viruses ... were actually a mixture of the genomes from different vaccine viruses," said Devlin, a lecturer at the University of Melbourne's School of Veterinary Science. "They actually combined, mixed together."

The viruses emerged in 2008, a year after Australia started using a European vaccine along with two very similar Australian vaccines to fight acute respiratory disease in poultry. The illness causes coughing, sneezing and breathing difficulties in birds, normally killing 5 percent of them.

The two new strains, however, were far more harmful, and since they were created have killed up to 17 percent of chicken flocks across Victoria and New South Wales, the two main chicken rearing states in Australia.

"What could have happened was one chicken was vaccinated with one vaccine and later was exposed to the other vaccine somehow, from nearby chickens," Devlin said.

Agricultural authorities in Australia have been informed of the results of the study, and are considering how to prevent similar cross-overs happening again.

"Use of only one vaccine in a population of birds will prevent different viruses from combining," Devlin said.

"Authorities are reviewing labels on vaccine to change the way vaccines are used and prevent different vaccines being used in one population."

(Reporting by Tan Ee Lyn; Editing by Daniel Magnowski)

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