There's no one correct answer here.
Marek's disease vaccine is given to day-old chicks, and helps prevent the tumors that kill chickens infected with Marek's disease virus. It doesn't prevent infections, but does prevent suffering and illness. Most 'backyard' chicken keepers only have this done.
People who show their chickens, and big commercial facilities, may have a list of vaccines done for their birds, and it's much more complicated.
Fowl pox vaccinations can be done in the face of a local outbreak.
Mary