What is says exactly, " depending on the strength of the virus, and the variety and strain of the chicken."
I couldn't copy/paste off the pfd...but that is the REST of the statement you left out....
According to the link I posted above, they are seeing a more virulent and vaccine resistant ( mutated) strain/strains of mareks, PLUS the vaccine is only 90% and some say can be as low as 75% affective AND does NOT stop the virus, but only the tumors. It DOES help build immunities in the chicks and may save a flock from dying, but does NOT stop them from contracting the virus OR being carriers of the virus.
Also, since chicks have no real immune system, which is why they can come down with cocci as day old chicks, if they are already immune suppressed, then that would also make them more susceptible to mareks...
There are NO cold hard facts in science! Viruses are mutating. Science is finding out things that they never knew before, with each passing day...which is why I recommended that thread, as some of the OP's are currently going through mareks in the flock and finding, from first hand experience, that the science and fact are not supporting each other!
Plus, you do realize that article was first printed in 1979? right?
Again, I'm not saying that this gal has mareks in her chicks, what I am saying is, it IS possible to have chicks with mareks...and even though the transmition through the egg is lower, it is not 100% negative either...just as the vaccine is not 100% affective...
Science is built on a theory, supported by many hypothesis...and not exact, like say mathmatics!!! Science is also, always changing!!! After reading that thread about OP's dealing with mareks, I can not say, with 100% surety, that day old chicks are totally safe from arriving, or getting mareks, or even being carriers...thus said...is it safer to order through a hatchery that vaccinates for mareks...heck yeah...does it stop the disease, nope...can the chicks get sick as young as a couple days old? I don't know, but would have to say, according to RECENT studies, yup!!!
Here is a hypothetical scenario, we have a pandemic, who is affect the most? The very young and the very old. Why? Because they are immunosuppressed. Does that stand true in livestock? Yes!!!! Is that 100%, nope.
Mareks is the most contagious, and most widely spread, chicken disease, ESPECIALLY in back yard, small flock settings. Why? Most people don't even know about it, or are misinformed, or have not kept up with current scientific findings and advancements. Another reason for the back yard flock to have such high rates of mareks is that most don't practice the methods that would help lower the amount of mareks in their flocks, like all in/all out, a closed flock, or biosecurity, to name a few examples!!! Lots don't even vaccinate for it or any other disease. Now that is another whole discussion, in and of itself, as there are many arguments for and against vaccinating vs breeding for immunity and I aint going there now!!!
As on OP put it, this disease is like a ghost, the vector for transmition is tremendous, for both the chickens and their owners...it is in the chickens dander and feathers, is airborne for several miles,, lives for very very long periods of time in the environment ect ect, and can be tracked on peoples shoes, clothes, hair, hands, equipment, feed bags, bales of straw and on and on....plus there is more than one stain of meraks...I can't remember exactly how many..but maybe 6 or so?
Bit I really can't remember exactly and new strains are either popping up, being found, or mutating....
1979? seriously? Whatever, I'm done...I will continue to research for myself!!! Thanks for the link!!!