Day after vaccine report

This is just for science and curiosity; I would be totally OK with WAY worse side effects.

The day after my first dose of Moderna, my arm hurt so much that I couldn't move my shoulder (luckily I was OK just moving my hand, so I could still type), and by late afternoon I felt generally like I had the flu.

The day after that (today), my arm still hurts but I have much more range of motion, the flu-like symptoms are gone, and I just feel a bit tired and like I'm recovering from some incredibly intense whole-body workout.

I feel good about my probable level of protection.

A Medical Mystery

Shows how spacey I am that I forgot to mention by far the most interesting part of my vaccine reaction. I'm having all-over body aches, but they're most severe and pronounced at the sites of previous injuries - including the foot I broke two years ago, which hasn't hurt at all for at least a year! It's at the exact site of the breaks, too. It currently feels like it did when I'd only had the cast off for a month or two.

What is the likely mechanism for this odd effect? I've only ever heard of old injuries being reactivated in scurvy, which I'm pretty sure I don't have.

Math help?

Hypothetically speaking, if the chance of a vaccinated person being able to contract a certain disease is 6 out of 100, and the odds of a vaccinated person being able to transmit a certain disease is also 6 out of 100, then what is the chance of one random vaccinated person giving that certain disease to one other random vaccinated person?

This is a math problem I need help with, not a factual statement about a disease. I'm trying to express a concept with actual numbers rather than just "very unlikely."