The vaccine doesn't prevent them from getting the disease.When he was a chick he got his shot for that but I don’t know how long it lasts.
He has no other symptom, no new hens, it happen months ago so if he has marks he would be dead by now.
he still the same jerk
It helps with "symptoms" and helps to keep them alive longer.
https://www.zoetisus.com/_locale-assets/mcm-portal-assets/msds_pi/pi/md-vac_cfl.pdf
This chicken vaccine makes its virus more dangerous | PBS NewsHour