I haven’t gotten vaccinated chicks, but if my “M” you mean Marek’s just know that it won’t prevent them from getting it. It may help them not die from it, but they can still contract it. There’s some talk about the fact that it doesn’t really help future stock, because if it does run through the flock – and it WILL at some point if they are in contact with any wild birds in your yard – those birds who become infected and survive and then hatch out chicks will pass on immunity. By the third generation, you have chickens who are likely immune to Marek’s and that strengthens the flock.
Marek’s went through our flock of 12 last summer when we moved them all to the big fenced yard to free-range as we normally do. But 5 were younger pullets hatched in the spring and we lost 4 of them.
We lost 3 others to other things (previous injury, fox, gave one away)
I bought 10 more chicks in September and have kept them separate from the older hens. They are now 22 weeks old and old enough to be exposed more safely, but integrating them at this age is going to be tricky so we’ll just wait until the weather is warmer and then do our best to minimize attacks.
We already had one bad meet-up a couple of days ago through a fence as a young one challenged #2 hen who pecked the beak so hard right in the middle at the top that she split it clear across from side to side just under the nostrils. We saved the beak so far and glued it (not a happy pullet) but we’ll see if she can keep it on and regrow it.
Marek’s is more lethal to pullets from 8-20 weeks. Mine got it at 16 weeks, it seems, and died or had to be put down within 2 weeks. We NEVER had a problem with it until last summer, but if you have free-ranging birds, they probably will be exposed to it at some point. If they are old enough, you probably wouldn’t even know because they have enough of an immune system to not even get sick.