https://www.valleyvet.com/ct_detail.html?pgguid=FBE23AC3-EB68-48DB-AB25-D274A6130A81I don't vaccinate my chicks.
I want strong genetics in my flock. So vaccinating doesn't help with that. I really want natural immunities.
The one vaccine I would give, salmonella, has been maligned by the FDA because it's ever so slightly different that the one used in Europe because of our regulations. I would give my chickens the salmonella vaccine were it a viable option but with no public support it's expensive, nobody's educated on it, it's not well-approved by the FDA and therefore it's largely not helpful to do.
Most diseases you can vaccinate for like Mareks the vaccine doesn't actually provide immunity it just hides the symptoms of an otherwise still-sick bird. In fact vaccinated but infected birds actually spread the disease more, not less.
Ultimately if the vaccine doesn't produce a sterile immune response I don't want it anywhere near my flock. That way I can know if my chickens are actually sick or not.