I think about 20% of my unvaccinated adults (most less than 8 months old) died , one every few months. Vaccinating exposed birds won't work. If I was showing, I would vaccinate my day old chicks because I'm sure quite a few chickens at shows are carriers. I have some links in my signature if you want some reading. If alamb is willing, you can send a dead bird for a necropsy to a state dept of ag vet, or a local vet can collect blood and you can send it to Texas A&M and they can test it. Terry, you could check your flock with a blood test of one. If you're negative, you have a good chance of the vaccinating work on the adults.