I’m not certain about the Shingrix vaccine itself, but typically reactions are either due to the components of the drug/vaccine itself (e.g. there may be some sort of irritant) or they’re due to the immune system’s response. For the latter, this can include things like T-cells and/or macrophages overreacting to the change in environment and signaling all kinds of swelling and inflammation, or even attacking otherwise healthy tissue temporarily. Your immune system’s general defense against invaders includes raising your body temperate (fever), and other things that can disrupt normal body functions.
The idea with things like that is yes, it’s uncomfortable for you, but if the invading pathogen has a narrower scope of livable conditions than you do, you can survive with a fever and it can’t. Differences in things like genetics, underlying conditions, existing allergies, and other factors can play into why different people experience different reactions to the same drug or vaccine.
thanks..I am going into it expecting the worst so as to be prepared and if I have no reaction then I will be pleasantly surprised