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 for by evolution. This is because such a severe 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. The vaccines inability to prevent infection and transmission allows the spread of highly virulent strains among vaccinated chickens. The fitness of the more virulent strains is increased by the vaccine. The evolution of Mareks disease due to vaccination has had a profound effect on the poultry industry. All chickens across the globe are now vaccinated against Mareks 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. References
External links

Mareks Disease Information at poultrykeeper.com
Vaccines could affect how the coronavirus evolves – but that’s no reason to skip your shot at The Conservation
Her relatives received prison terms, the parents dying in jail. In 1988, members of the Carr family from Alturas, Polk County, Florida, fell ill from what appeared to be thallium poisoning. Peggy Carr, the mother, died slowly and painfully from the poison. Her son and stepson were critically ill but eventually recovered. The Carrs neighbor, chemist George J. Trepal, was convicted of murdering Mrs. Carr and attempting to murder her family, and sentenced to death. The thallium was slipped into bottles of Coca-Cola at the Carr and Trepal homes. Thallium was the poison of choice for Saddam Hussein to use on dissidents, which even allowed for them to emigrate before dying. In 1995, Zhu Ling was the victim of an unsolved attempted thallium poisoning in Beijing, China. In 1994, Zhu Ling was a sophomore studying physical chemistry at Tsinghua University in Beijing. She began to show strange and debilitating symptoms at the end of 1994, when she reported experiencing acute stomach pain and extensive hair loss. Ultimately she was diagnosed on Usenet with poisoning by thallium. To this day, there is still speculation among Chinese expatriates overseas as to the poisoners identity. The only suspect of the police investigation, Sun Wei, is a member of a family with high-level political connections, which may have been used to halt and suppress the results of the investigation. Sun Wei was Zhu Lings classmate and roommate in Tsinghua University from 1992–1997. Tsinghua University also said she was the only student who had access to thallium compounds at the school.