https://en.wikipedia.org/wiki/Marek%27s_disease#PreventionTL:DR
When a vaccine is leaky, meaning it doesn't totally prevent infection or transmission, just lessens symptoms, it still allows the virus to replicate and mutate in vaccinated hosts, but more virulent mutations are selected for.
Marek's disease originally had a low mortality rate in unvaccinated chickens, after the leaky vaccine and generations of vaccine-resistant mutations, the virus now has a near 100% mortality rate in unvaccinated chickens.