"The outbreak began as early as the first half of 2003, probably in China," the British scientific weekly says, citing health experts whom it did not identify.

"A combination of official cover-up and questionable farming practices allowed it to turn into the epidemic now under way."

The report is carried in next Saturday's issue of New Scientist, a copy of which was made available to the press on Wednesday.

It coincided with a crisis meeting in Bangkok of Asian countries battling an epidemic of the dangerous virus among their poultry stocks, and China's confirmation that it had recorded outbreaks of it on farms in three provinces hundreds of miles (kilometers) apart.

New Scientist says the suspected cause for the widespread dissemination of the H5N1 virus was mass vaccination of poultry flocks by Chinese farmers.

Worried after all chickens in Hong Kong were slaughtered to curb an outbreak of the disease in 1997, Chinese producers started vaccinating their birds with an inactivated H5N1 virus.

"This may have been a mistake," New Scientist says, pointing to the impact from small genetic mismatches that can occur between vaccines and the notoriously mutating flu virus.

The slight mismatch meant that Chinese birds would not be primed to destroy the specific form of the virus with their immune systems.

Instead, they could still harbour the virus but show no symptoms, and so could pass it on in to other flocks when they were traded.

"If the vaccine is not a good match for the virus -- as is the case with the H5N1 strain now sweeping Asia -- it can still replicate but most animals do not show signs of the disease.

"In this way, the intensive vaccination schemes in south China may have allowed the virus to spread widely without being spotted," New Scientist says.

The strains causing outbreaks in South Korea and Vietnam are very similar, and analyses of strains from other countries are still being analysed.

However, "it looks as if all the outbreaks started with the large-scale distribution of one strain" that appears to have originated in China, the report says.

It added that illicit trade in poultry helped propagate the spread in Southeast Asia.

The H5N1 subtype of bird flu is highly virulent for poultry flocks and has been found to be contagious, and potentially lethal, for humans.

It is transmitted from fowl to humans but cannot be passed from humans to humans.

The worry is that this strain could merge with a conventional human flu virus, combining both lethality and contagiousness.

China was fiercely criticised last year after it was found to be the source of Severe Acute Respiratory Syndrome (SARS), which leapt to several neighbouring countries and then to Canada, sparking a global scare in which 800 people were killed and 8,000 sickened.

The outbreak -- which may have begun from small caged mammals, considered a delicacy in southern China -- was initially covered up by the authorities, although their later response drew praise from health agencies.

According to the World Health Organisation (WHO) website, flu pandemics originated in China in 1957, 1968 and 1977.

It points the finger at China's village farming practices in which ducks, pigs and humans live in close proximity.

Pigs can carry both avian and human influenza viruses and thus can act as the "vessel" in which different strains can meet and swap genes, creating a new, killer strain that is then passed on to villagers.

TERRA.WIRE