DD1 reacts really badly to the idea of vaccinations, and we had to get a set of 3 top-ups when we moved here (NZ) to keep up with the local system.
After a dreadful reaction (sobbing screaming etc) for the first one, I decided that leading her into it gently was the best plan - bad idea. I told her the night before: she cried herself to sleep, she sobbed in the waiting room, screamed when the nurse came in. It was horrible. But frankly it had to be done, and on a pretty short time scale for starting school. So I sat her on my knee like they have you do with toddlers, held her arms by her sides in a hug (again like you do with toddlers) and the nurse gave it to her really quickly.
3rd one, I didn't tell her in advance, but she knew as soon as we went into the doctors. Similar crying and shouting in the nurses office, but was a little easier to contain.
It wasn't pleasant for any of us, but frankly, having to put her through the stress of trying again another day was just too much.
I was full of praise, and treats afterwards (and had arranged for DD2 to be elsewhere so I only had to concentrate on DD1), and I definitely think that the "hug hold" helped DD1 a bit too, as she certainly leant into me a bit for the 3rd one.