Vaccinated chickens can be carrier of a disease without getting sick from the virus it’s vaccinated for.
I don’t know much about it other than I read from people who get sick chicks if they mingle vaccinated with not vaccinated. Especially Marek can get nasty that way.
If I've read the research correctly, vaccinated birds are not "carriers", but that the vaccine can be "leaky", meaning that an unvaccinated bird can transmit Marek's to a vaccinated bird (who may not have symptoms) and then that vaccinated bird can pass Marek's to unvaccinated birds.
From The American Poultry Association
https://www.amerpoultryassn.com/2022/07/mareks-disease-vaccination/
"Vaccinated birds and unvaccinated birds can be put together, but you need to be aware that although the virus used for the vaccination will not cause disease in unvaccinated birds, the vaccine is “leaky,” which means vaccinated birds can still get the chicken version of Marek’s from infected birds. The vaccine helps prevent the bird from developing symptoms and tumors, but a vaccinated bird who has gotten a chicken form of Marek’s can pass that along to an unvaccinated bird.
In the past, a particular study has often been quoted to discourage use of the vaccine by implying that the vaccinated birds may cause more mutations in the virus, resulting in vaccines that don’t work and a worse variant of Marek’s. Thankfully a more recent study can calm our fears about that worry. This study found that vaccinated birds actually help decrease both the amount and the severity of Marek’s in their unvaccinated coop mates. Vaccinated birds are protecting unvaccinated birds to some extent. We don’t know if vaccinated birds actually pass some of the harmless turkey virus on to other chickens, since there has not been a documented case of that. Or it could just decrease the viral load on the unvaccinated birds by keeping infection to minimum. But we do know that the leaky nature of the vaccine is not causing the mutations that earlier studies worried about."
Here is a research article on a similar topic:
https://pmc.ncbi.nlm.nih.gov/articles/PMC7058279/#:~:text="Leaky" vaccination of chickens infected,is rarely taken into consideration.
"Here, we use transmission experiments involving Marek disease virus (MDV) in chickens to show that vaccination with a leaky vaccine substantially reduces viral load in both vaccinated individuals and unvaccinated contact individuals they infect. Consequently, contact birds are less likely to develop disease symptoms or die, show less severe symptoms, and shed less infectious virus themselves, when infected by vaccinated birds. These results highlight that even partial vaccination with a leaky vaccine can have unforeseen positive consequences in controlling the spread and symptoms of disease."
"Leaky" vaccination of chickens infected with Marek“s disease virus reduced symptoms and lowered mortality in the unvaccinated contact birds infected by them. The cause was transmission of lower virus loads from vaccinated than unvaccinated birds; this benefit of leaky vaccination is rarely taken into consideration."
