BruceC, yes, I have heard of that before, several years ago, but believe it was a person who wasn't even traveling on the Amazon, just getting off in Rio as was I. Seems it was not officially required there, except by Regent. I got it anyway. Next trip, I did the Amazon, and so it was still valid as I would have needed it there. I had no reaction to the shot whatsoever.
Please let us know what you find. There is not really clarity on this issue from this board as I remember for those going on the Amazon. You will need to deal with Regent on this, and might ask your TA to inquire.