That was a really good article. He is talking about a different thing than what I was wondering...  about live virus vaccines. It's been 30 years since I've studied this stuff, and I doubt that they even use live vaccines anymore. In a live virus vaccine, the vaccine itself can actually be the source of the disease spreading because you are in effect giving the animal the virus, although in a weakened form. Most vaccines use inactive (dead or otherwise altered) forms of virus, so the animal will make the antibodies, but there is no living virus introduced into its body, so no danger of shedding virus from a vaccinated animal that was otherwise healthy.
This article is talking about Mareks vaccinated chickens becoming a reservoir for the virus. The vaccinated bird can fight the disease enough to stay alive, but not enough to kill the virus completely, so they spread the disease.  If your flock is not vaccinated, and they are exposed to one of the more virulent strains, they all die and the disease spread is stopped. A virus that kills its host too quickly will not survive in nature...  it needs to have a new host to keep living and reproducing. Hot or virulent strains are not spread much in nature because the host bird dies before it can pass the virus on to others. They aren't seeing the hot strains in wild bird populations long term. Vaccinated birds break the natural cycle, artificially keep a bird alive that should be dead, and turning it into a walking virus factory that spreads the virus to even more birds (mostly vaccinated now) so a monster virus, that nature would have made very rare or extinct, is flourishing. We have become dependent on the vaccine, which is never a good thing...
But the vaccine itself is not the source of the virus, it just creates walking incubators for a fatal virus and allows it to become even stronger.
I remember when Nambroth was struggling with Mareks, and it seemed to me that stopping all vaccines was the best way to go. SOME chickens will survive, and you start over with natural immunity, protection you need from wild bird carriers. I think we have already passed the point of no return. It's a catastrophe waiting to happen. Best bet is to create a non "leaky" vaccine that actually kills the virus.