https://en.wikipedia.org/wiki/Marek%27s_disease

TLDR: Vaccine is leaky (The vaccinated does not have symptoms but can transmit). Virus becomes more contagious and deadlier. Chicken that don't take the vaccine are almost guaranteed to die. Chicken industry uses vaccine as weapon against other chicken farms to kill off competition.

"However, because vaccination does not prevent infection with the virus, Marek's is still transmissible from vaccinated flocks to other birds, including the wild bird population. The first Marek's disease vaccine was introduced in 1970. The disease would cause mild paralysis, with the only identifiable lesions being in neural tissue. Mortality of chickens infected with Marek's disease was quite low. Decades after the first vaccine was introduced, current strains of Marek Virus cause lymphoma formation on throughout the chicken's body and mortality rates have reached 100% in unvaccinated chickens. The Marek's disease vaccine is a leaky vaccine, which means that only the symptoms of the disease are prevented"