I'm having problems with Marek's at the moment and I'm wondering if I can vaccinate my remaining chickens? They are a few years old to a few months old. I'm also wondering if it is safe to vaccinate again. The lady at the feed store said she didn't know if the chicks that we bought from them were vaccinated or not, if they were is it alright for me to vax again? Also would it help my sick chickens if I vax them? Or could it make them worse?