[fer]

From your link:

> Chickens become infected with MDV by inhalation of dust contaminated with virus shed from the feather follicles of infected birds. In a contaminated poultry house, chicks are infected soon after hatching and remain infectious for life

A "leaky" vaccine isn't a problem in itself if it helps clear the infection faster, because in fact that means there will be fewer chances for mutations. Now in a massified poultry industry, that vaccine only helped chickens not die while remaining infectious, and that's obviously turning farms into mutation making machines.

The Marek scenario applies to very few diseases.