Long term effects are highly likely due to IgE sensitisation when any foreign protein is introduced into the skin. The immune system sees this as a parasitic invasion and becomes sensitised to any other protein which is within 1 amino acid difference. But the main problem with mRNA vaccines is that they are highly unstable, and early quality control studies showed that mRNA is degraded, randomly altered, in 30% of doses. This was under perfect storage conditions, but considering that in the field mistakes happen, protocols are not always followed, the likelihood of being injected with a randomly altered mRNA becomes quite high.

So if you have any mutations that match the sensitised protein (be it the intended one or a randomly degraded one) you could develop an auto-immune disease and possibly cancer. In short, we do not know because we do not have long term human studies, or even animal studies.