The basic problem is that, in practice, it is difficult to actually immunize 100% of a flock. This raises the specter of “silent spread” in which immunized birds carry the disease without experiencing symptoms. The following is from The Herald:
‘Silent spread’ occurs because immunised birds can still catch and pass on avian flu, but without presenting any symptoms. Therefore, as protection levels rise in a vaccinated flock, it becomes ever harder to detect the spread of avian flu, simply because fewer birds die. The result is increasing amounts of bird flu virus contaminating the birds’ surroundings without farmers realising it.
Hard to find good news on this front.