Copypasta: Vaccination is the only known method to prevent the development of tumors when chickens are infected with the virus. However, administration of vaccines does not prevent transmission of the virus, i.e., the vaccine is not sterilizing.[3] However, it does reduce the amount of virus shed in the dander, hence reduces horizontal spread of the disease. Marek's disease does not spread vertically. Before the development of the vaccine for Marek's disease, Marek's disease caused substantial revenue loss in the poultry industries of the United States and the United Kingdom. The vaccine can be administered to one-day-old chicks through subcutaneous inoculation or by in ovo vaccination when the eggs are transferred from the incubator to the hatcher. In ovo vaccination is the preferred method, as it does not require handling of the chicks and can be done rapidly by automated methods. Immunity develops within two weeks.[4] 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.[11] Infection of the host and the transmission of the virus are not inhibited by the vaccine. This contrasts with most other vaccines, where infection of the host is prevented. Under normal conditions, highly virulent strains of the virus are not selected. A highly virulent strain would kill the host before the virus would have an opportunity to transmit to other potential hosts and replicate. Thus, less virulent strains are selected. These strains are virulent enough to induce symptoms but not enough to kill the host, allowing further transmission. However, the leaky vaccine changes this evolutionary pressure and permits the evolution of highly virulent strains.[12] The vaccine's inability to prevent infection and transmission allows the spread of highly virulent strains among vaccinated chickens. The fitness of the more virulent strains are increased by the vaccine. The evolution of Marek's disease due to vaccination has had a profound effect on the poultry industry. All chickens across the globe are now vaccinated against Marek's disease (birds hatched in private flocks for laying or exhibition are rarely vaccinated). Highly virulent strains have been selected to the point that any chicken that is unvaccinated will die if infected. Other leaky vaccines are commonly used in agriculture. One vaccine in particular is the vaccine for avian influenza. Leaky vaccine use for avian influenza can select for virulent strains which could potentially be transmitted to humans.[13] * * * If you have a better more digestible summary, I'm all ears.