Vaccines used by China on birds to protect them from avian flu virus may have led to its spread, a report in The Times said.
The report quoting health experts of the New Scientist magazine said the epidemic could have begun as early as the first half of 2003. According to the experts the outbreak could have spread through a combination of 'cover-up by China' and certain poultry practices.
Chinese poultry farmers used a vaccine based on a strain of the virus -- H5N1 -- after an outbreak had triggered a mass cull in Hong Kong in 1997.
Experts now say the vaccination might have been a mistake as the vaccine was an imperfect match for the current strain. The faulty vaccination, they add, might have suppressed the symptoms in birds allowing the epidemic to spread.
The magazine also quoted experts who suggested the epidemic originated in China and then spread through migratory birds.
The WHO is opposed to vaccination as a means to control the virus, preferring to cull birds in order to halt its spread.
But China has denied the charge. Zhang Qiyue, a foreign ministry spokeswoman, said: "We believe that such an allegation is totally inaccurate, groundless and doesn't respect science."
Allegations of a cover-up are embarrassing for China, still smarting from international criticism of its cover-up of SARS outbreak.
China admitted the outbreak only last week when it had to cull up to 200,000 birds and quarantine nearly 40 farmers who had been in contact with the affected birds.
China also said it would halt exports of poultry from Guangxi, Hubei and Hunan provinces, which have confirmed outbreaks, while banning imports from Indonesia, Pakistan and Laos.