Hi SF,Funny, I had no reaction but my buddy who is 68 just got his first shot and he said it kicked his ass for about three days as well. He described flu like symptoms.
The new (post 2017) Shingrix vaccine contains no live virus (a recombinant virus that is missing key genetic material) and included molecules that stimulate the immune system. [The older vaccine, Zostavax, contains a live, but weakened virus.] Therefore, if you do suffer flu-like symptoms after a Shingrix vaccination, it is isn't because you are infected but because your body is reacting, as it should, as if you did have an infection. Hence, the flu-like symptoms that some folks experience after a Shingrix vaccination reveal the "costs" of just your own immune system in trying to end a flu (or faux shingles / chickenpox) infection. When you have a serious infection, like the flu, the body dedicates huge resources to fighting it off (and diverts resources from many normal housekeeping activities, such as bone and muscle growth).