https://en.wikipedia.org/wiki/Marek's_disease#Prevention

However, because vaccination does not prevent infection with the virus, Mareks 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.

Anyone know if the CCP Vax also leaks?