Bairo, an important thing to remember about the Marek's vaccine is that it does NOT guarantee against Mareks.
Also, it's really only effective on chicks. So you risk the adult flock if you start vacinating chicks. There are some that say it does not shed, but I don't see how it can't if there's no gurantee it can prevent Mareks.
See my point?
The rogue sparrow is a one-in-a-million thing.
If you innoculate, YOU may be the rogue sparrow. (lmao at that visual, btw)
I am not a professional in any way, so make your own decisions please. I've just read up on this and talked to various breeders. My decision is to not vaccinate for anything, and only use antibiotics on injured birds. Sick birds are isolated, and if they don't rally on their own (with very good care and extra vitamins in their feed), they are culled. I want the strongest flock possible as naturally as possible.
This attitude has cost me some gorgeous, SQ birds. But I also know that I'm breeding from the strongest stock possible. I had a little bit of a battle with Mareks early on, but I am delighted with my current flocks of birds! They're tough little boogers, and adorable to boot