The health chief says it is too early to decide whether Hong Kong should switch from using a Dutch-made bird flu vaccine to one from the mainland. Secretary for Food and Health York Chow Yat-ngok said University of Hong Kong test results on the efficacy 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. There have since been calls for the government to consider switching to a mainland-made H5N1 vaccine. Some of the birds that died at the Yuen Long farm had been vaccinated against bird flu - giving rise to fears that the virus may have mutated, and that the vaccine might be losing its effectiveness. The farm outbreak came six months after Hong Kong detected the virus at four wet markets in June, prompting a 21-day shutdown and a ban on overnight stocking of live chickens in markets. Sources said the strain was most probably H5N1 as it was deadly and virulent. Definitive test results are not expected for days or even weeks. 'As you know, the virus that has been detected on some parts of the mainland is different from that we have detected here. So we have got to first make sure that the vaccine is effective,' Dr Chow said. If a decision was made to use a mainland vaccine, it would be tried on selected farms first, he said. Dr Chow said he had asked mainland authorities to help test and compare the mainland and Dutch vaccines. He said his bureau gave virus samples from the June outbreak to the University of Hong Kong. 'They have been conducting some tests on chickens, but because of the difficulty of finding a totally ... 'clean' chicken without any immunity [to the vaccine], so far they have not been conclusive,' he said. 'And they said they would need a bit more time.' This appeared, however, to contradict statements made on Tuesday by the university's head of microbiology, Yuen Kwok-yung, who said the investigation had found that vaccines used locally were failing. Dr Chow added that the current outbreak was 'well controlled'. 'So far, we have not seen any sign of it spreading,' he said. The World Health Organisation said the outbreak in Hong Kong was 'disappointing but not surprising'. 'Vaccination remains a WHO recommendation,' Peter Cordingley, a spokesman for the WHO Western Pacific regional office, said. 'The virus is established in many countries. It remains a serious threat to public health. Governments should have good surveillance systems in place and be prepared for the possibility of a pandemic.' What's at stake 31 local chicken farms with total capacity of 680,000 chickens 86 wet market stalls and 47 fresh provision shops permitted to sell fresh chicken 5.8 million day-old chicks imported from the mainland last year; more than 2.6 million so far this year Since Tuesday Culling of 68,000 birds at infected index farm in Lau Fau Shan, Yuen Long A second uninfected farm quarantined and 12,000 poultry being culled Culling of 10,000 birds at Cheung Sha Wan wholesale market yesterday Chickens will be kept at 29 unaffected farms for the next 21 days Source: Hong Kong government