There are problems with the Marek's vaccine because it is leaky and it doesn't necessarily prevent infection, so it can actually be harbouring the disease and allowing it to mutate. It may be why more virulent strains are becoming prevalent, particularly in The States. Here in the UK we don't have the big hatcheries that you guys order from, so most people buy chickens from small breeders or auctions where they are not vaccinated and the Marek's here seems to be mostly lower grade. Personally as someone with a Marek's infected flock, the vaccine is not something that I would be interested in, but everyone has to make up their own mind on these things. I'm not anti vaccine in general, but I'm not sure this particular one is the best option.