‘LEAKY’ vaccines that manage disease but fail to prevent infection and spread are more effective than thought in controlling Marek’s disease, new research has revealed.

The virus has a significant impact on poultry production. It can lead to chickens developing tumours in various parts of the body, eye cancer, and wing and leg paralysis, eventually leading to death.

It can also have an impact on egg production and quality and is estimated to cost the US poultry industry US$1bn a year.

Live vaccines are the most common way to control Marek’s.

Before this research, little was known about how they impact on overall bird populations, particularly in unvaccinated chickens.

Scientists at the Roslin Institute, in collaboration with researchers from the US Department of Agriculture’s Avian Disease and Oncology Laboratory (ADOL), tested the impact of vaccination on Marek’s disease transmission.


One group of chickens received a leaky vaccine – this contained a related live virus originating from turkeys, which causes an immune response but no symptoms.

A second group received a sham vaccine, which contained no biological material. Both groups of birds were then infected with Marek’s disease virus.

Groups of these infected birds were placed with sets of unvaccinated chickens, and more than 97% of birds became infected.

However, unvaccinated chickens that had contact with vaccinated birds were less likely to develop full-blown Marek’s disease, and there were fewer deaths. This was found to be because vaccinated birds transmitted fewer copies of Marek’s disease virus.


The Roslin Institute’s Richard Bailey said: “We found that leaky vaccines can provide benefit in terms of reducing the presence and severity of symptoms, and mortality, caused by Marek’s disease even for unvaccinated chickens.”

The study was published in the journal PLOS Biology.