Hatchery birds are not vaccinated unless you order them yourself and pay the extra to have it done. Feed stores do not pay that extra money.
Silkies are more susceptible and you usually start seeing problems between 9 weeks and six months. Females seem slightly more prone than males.
It's a one time vaccine and must be administered within 24 hours of hatch.
Yes I've had an issue with it before and it killed off all my younger silkies that year. None of the ameraucana or EEs were affected. I vaccinate now because it's heartbreaking to watch. The silkie flock I have now have never had issues with it, but I will continue to vaccinate them even with it costing me $25-30 a hatch to do so. The vaccine must be used within a couple hours.