Scientists in Australia have found that a vaccine used to prevent a respiratory disease in chickens has combined with another vaccine to form a deadly virus.
They say the new strain is much more virulent than the disease itself and has so far killed up to 17% of chicken flocks in South Australia.
In a paper published in the journal Science scientists say the creation of the deadly new variant was made possible only because the two vaccines contained live viruses.
The discovery has prompted authorities to review vaccine use in animals.
Live vaccines which have a weaker version of the virus are quite commonly used in animals.
Meanwhile New Zealand's Poultry Industry Association, which represents the country's 260 commercial meat chicken farmers, says there is no risk to the domestic industry.
Executive director Michael Brooks says New Zealand chicken farmers use only one vaccine, and it comes from the United States.
"We do vaccinate on occasions if their is a potential issue here, but it is pretty rare in New Zealand."
He says New Zealand is unique in that it has very few poultry diseases and none of the major ones.
"That's the reason we have import barriers to raw poultry and eggs coming into New Zealand," Mr Brooks says.