The Mexican vaccine being used to inoculate birds at Pak Sha farms has yet to be proven effective against bird flu in Hong Kong, an expert cautioned. Disease expert Dr Robert Webster of the St Jude Children's Research Hospital in Tennessee, said the difficulty with the vaccine was that no international code had been set by agricultural authorities for the use of 'killed vaccines' on animals. Killed vaccines are preparations of a whole virus that has been inactivated by a chemical. By contrast, human vaccines have to meet strict international standards. A spokeswoman for the Agriculture, Fisheries and Conservation Department said: 'The vaccine is an inactivated vaccine made in Mexico. Quality control is done in the Netherlands. The department has sought the approval of the Pharmacy and Poisons Board of Hong Kong to import this vaccine.' Dr Webster said: 'You must standardise a vaccine to see that it contains enough antigens - substances stimulating antibodies in the blood - to be useful.' His concern was that the Mexican vaccine, an H5N2, had yet to be shown to be effective for H5N1 viruses.