The government is being urged not to rush to switch vaccines, as the H5N2 vaccine used to inoculate poultry at local farms has been shown to have prevented bird flu for the past 10 years in high-risk areas such as Mexico and Italy. On Wednesday, Secretary for Food and Health York Chow Yat-ngok said University of Hong Kong test results on the usefulness of the Dutch vaccine on virus samples taken at four wet markets in June were 'not conclusive'. Hong Kong has been using a H5N2 vaccine made by animal health care company InterVet from the Netherlands since the government started a vaccination programme for local farms in 2003. InterVet's vaccine appeared to be effective until the H5 virus killed 200 chickens at a farm in Yuen Long on Monday and early Tuesday. Animal virologist Frederick Leung Chi-ching of the University of Hong Kong said there was 'no proof' that the vaccine programme was not working. 'If anything, I would in fact argue that the vaccine programme is working fine,' he said. He said animal experts had been asking the government since 1999 to add the vaccination programme to its bird flu control plan. 'The last five years' record of using vaccine is the proof of that. How one uses the vaccine is also very important as to the effectiveness of the vaccination programme,' he said. He added that countries such as Mexico and Italy had been successfully using the vaccine to combat bird flu, including H5N2, for more than 10 years. Professor Leung said he was not convinced though that the government had established through research 'the source of H5N1'. 'I believe we need to change our thinking about the virus and an appropriate policy in respect of culling chickens,' he said. He added Hong Kong still needed to observe good farming practices and personal hygiene. Ornithologist and environmentalist Martin Williams said he was glad the government had not shut down Mai Po nature reserve. He said wild birds in Mai Po had been 'scapegoats' of bird flu outbreaks in Hong Kong. He also did not think there was a need to shut the aviary in Yuen Long. But Dr Williams said this could provide 'minor protection ... by stopping humans carrying the virus to the aviary'. He had not noticed signs of 'silent epidemics' although 'vaccinated birds took more time to die in the Hong Kong case'. A spokeswoman for the Agriculture, Fisheries and Conservation Department yesterday confirmed that test results had shown the chicken deaths at the Yuen Long farm this week were caused by the H5N1 virus. But what clade it belonged to was still subject to genetic testing, which could take weeks or months, she said. In the last scare at four wet markets in June, genetic sequencing showed the H5N1 was still the 'clade 2.3.4' - or Fujian strain, which first emerged in Fujian on the mainland - which had been the dominant type for Hong Kong bird flu.