That’s a good question, and sorry for my late response. Looking back at the EMA report on the Moderna vaccine, it seems that traces of mRNA in the brain are gone after about 25 hours. (I must have missed this information previously.) I guess this means that mRNA entering the brain is not a significant problem, although I’m not sure if the same applies to the Pfizer mRNA vaccine. It would also be a different story if the mRNA that managed to enter the brain gets translated to spike proteins therein, but there is no evidence of this for now, as far as I know.

Still, looking back at the past few months, there has been no signal of mRNA vaccine danger (unlike the DNA-based ones from AstraZeneca and Johnson & Johnson that can cause vaccine-induced thrombotic thrombocytopenia; VITT). So, if I have to bet, I would say that mRNA vaccines will not cause any long-term neurological problems.