0wen
Songster
A lot of information that says both really. Primary school of thought is that the chicks need to be vaccinated before they are exposed to the virus for it to be effective. This translates to most people hatching day 1 to be sure they beat exposure. I've vaccinated chicks that were a week or so old, maybe as old as 9 days - before they were exposed to anything from outdoors. If you follow the links in my sig, it has some conversation on it. I routinely vaccinate live chicks when they arrive in the mail (as much as 5 days old). It's difficult to measure success because I've not lost a bird like this. So, despite Marek's being identified in my region, I could just be lucky, or it could be biosecurity and vaccination efforts paying dividends.
Also, I've seen a study (possibly linked in one of the topics in my sig links) suggesting that vaccinating birds AFTER symptoms have become present has had some success in stopping/slowing/reversing symptoms - this is all second hand, so obviously accounting for it is impossible, but if you have a bird that's showing signs - death is soon to follow, so why not give it a try?
Also, I've seen a study (possibly linked in one of the topics in my sig links) suggesting that vaccinating birds AFTER symptoms have become present has had some success in stopping/slowing/reversing symptoms - this is all second hand, so obviously accounting for it is impossible, but if you have a bird that's showing signs - death is soon to follow, so why not give it a try?