Since the vaccine is supposed to be given a day or two after hatch, and this is because it needs to be given before contact with the virus. If someone does not have the virus, I would assume that it would give them protection as well. Common sense, right?
Sort of! It is much, much better than nothing. But, the reason the vaccine gives more protection to day-old chicks (when they are vaccinated properly and not exposed for a bare minimum of 2 weeks) is that a newborn Chick's immune system is still developing and, by the nature of immune systems, is "looking" to develop based on the natural flora (viruses and bacteria) it is exposed to via momma hen those first two days... When the chick would be staying very close to mom and still mostly living off of yolk. That's why the vaccine is thought to give less protection to older (unexposed) chicks. It may also be why the "booster" at two weeks old helps some birds that are going to be exposed.