Less-than-perfect vaccines create a 'leaky' barrier against the virus, so vaccinated individuals sometimes do get sick, but typically with less-virulent symptoms. Because the vaccinated individuals survive long enough to transmit the virus to others, the virus also is able to survive and to spread throughout a population. "In our tests of the leaky Marek's-disease virus in groups of vaccinated and unvaccinated chickens, the unvaccinated died while those that were vaccinated survived and transmitted the virus to other birds left in contact with them," Nair said. "Our research demonstrates that the use of leaky vaccines can promote the evolution of nastier 'hot' viral strains that put unvaccinated individuals at greater risk."
I read this and it made me think of you guys/girls. Anyway this so sounds like what happened with flu in poultry stocks (add in side effects from monoculture, bad biosafety practices).
We never make a perfect flu vaccine - we go for max protection for the mass but it is too variable to really stop it. We do the same or worse for poultry and as long as there is enough of that around it might promote nastier avian variants. (How well they can jump to humans is another issue).