The source of the bird flu outbreak on Lantau Island remained a mystery last night as the government urged people not to panic over the latest cases, saying there was no sign of a large-scale outbreak of the disease. The Food and Environmental Hygiene Department was trying to determine whether the chickens which died at a Mui Wo stall on Wednesday were imported from the mainland or locally raised. Amid news that the outbreak had spread to four wet markets across the SAR, Ingrid Yeung Ho Pui-yan, acting Deputy Secretary for Health, Welfare, and Food, said the government would introduce a new vaccine to all 81 chicken farms in Hong Kong to stop avian flu outbreaks. Twenty-two chickens died at the Mui Wo stall, and the owner had to slaughter the remaining 60 chickens. Subsequent tests revealed they carried the H5 avian influenza virus. A strain of the disease killed six people in the territory's first bird flu outbreak in 1997. The Mui Wo stall was shut down after being thoroughly cleansed and disinfected by departmental staff. At Mui Wo market, a chicken stall holder next to the infected one said business was normal. 'I sold 20 chickens this morning; that's about the same as any other day. People who buy chickens are already used to this sort of news,' said stallkeeper Lau Shiu-hai. Ms Lau said she cleaned her stall at least twice a day. 'The weather is not very stable in the past few days. One day it's cold and another day it's warm. It is very easy to catch cold in such weather,' she added. Shopper Wong Pak, 61, said he was not afraid of bird flu and would continue to eat chicken. 'I will wait for a few days but I'm not scared. There are many important festivals coming up so you will have to buy some chickens - that is tradition. I like fresh meat,' he said. Another customer, Lawrence Lau, said he would only buy frozen chicken because it was more convenient and safe. The chicken deaths came two weeks after 31 ducks, geese and swans died from H5 flu in Penfold Park at Sha Tin racecourse. Sixty birds were also slaughtered and the park was closed.