You can, although it may or may not do any good. It is ideal to give it to them before they can be exposed to it in the environment, that is why they say to use it on chicks. If your chickens have already been exposed to it ( which is impossible to tell ) then it may not help, but if they have not been exposed then it may. The vaccine does not truly stop the expose to the virus but stops the tumors from forming that kill the chicken. The makers of the vaccine are very nice and helpful. You can call them also to explain it better....
I don't know. The vaccine comes in large quantities ( 1,000+) and i think that you might be able to vaccinate the chickens. Maybe a veterinarian could help you.
I also have read that if you give the vaccine to a bird that is already exposed it might prevent the tumors from developing.
When you buy the vaccine you will get enough to vaccinate chickens for the rest of your life. Unfortunately once you use the vaccine you can't save what is left over. It is about 18.00 plus shipping. Some might think that it is not cost effective to do just a few, but if it keeps them from getting sick it it worth it, IMO. I spend many times that trying to save one from Markes.
Jeffers online had it for like $9-$12. You can take the dried waffer and divide it into 4. Then take the dilutent and divide it into 4 in a separate glass. Add the 2 together and you still have 3 doses left. I have also read that it can be helpful to give it to those affected....