It makes a huge difference for me after the chickens got vaccination. My most unvaccinated chickens died from Marek within first two years. None of my vaccinated one died yet (except two killed by a fox). They do have respiration disease too, but it's not so fatal. Water is the only one possibly have Marek after vaccination.So sorry to hear that the Selenium and vitamin E didn't work. I lost 2 grown hens to illness this summer, as well as some young pullets and just last week one of my favorites just flat out disappeared. I assume it was a hawk -- she was my smallest (a Mille Fleur D'Uccle.). I am rather heartbroken with all the losses. Mine all have names and are considered family members.
I personally am not seeing a different loss (from disease) ratio between the Marek vaccinated and the unvaccinated ones. Maybe I have something else going around.
My coop is in wood area, so hawk never got any my chicken. I have electric fence around their run, that stopped fox.