From Time magazine:
This week's outbreak is particularly alarming because some of the chickens killed by the virus appear to have been vaccinated against it, according to the government. Lo Wing-lok, president of the Hong Kong Medical Association and an expert on infectious disease, says that Hong Kong uses an older version of the H5 vaccine than mainland China, where there are more frequent outbreaks and farmers vaccinate poultry specifically against the H5N1 strain of the virus.
While the older, umbrella vaccine has proven effective in the past, Dr. Lo says it may be losing its potency as newer strains of the virus spread around the region. A new study by University of Hong Kong microbiologists of a virus outbreak earlier this year found that chickens that had been vaccinated against the H5 virus had only one-quarter as many antibodies as those vaccinated in 2001. Yuen Kwok-yung, head of the University's microbiology department and an expert on avian flu, warned recently that the vaccine may be approaching total failure.