Though at least with pain, you could compare with previous reports from that same person. Pain started day 1, never subsided until 2 months after. Pain starting at 2 months is probably not a side effect, unless it's something that consistently shows up in the data after the same period of time since vaccination (background noise would not).

Maybe add a self-reported intensity scale for the symptoms.

Anyway, I can see why studies like these can be hard.