(Personal communication from Christopher Shaw PhD, with his permission. He references the only paper I am aware of on spread of mRNA vaccines using lipid nanoparticles, available here: https://sci-hub.se/10.1016/j.ymthe.2017.03.035
In a separate communication, Dr. Ulm commented on this paper: “ There have been some biodistribution studies, but questionable if the LNP formulation used corresponds to what's actually in the vaccines, which is a crucial point since the formulations vary so significantly”.)
We don't know that the mRNA construct in its lipid coat gets past the blood brain barrier (BBB) in humans, but it seems a reasonable concern given that it clearly does in animal studies carried out by Bahl et al in 2017. Bahl was with Valera, a subsidiary of Moderna and they clearly found mRNA from the vaccine (this was not a Covid-19 trial but for several flu strains) in the CNS. True, there wasn't much compared to the muscle site of injection, but that there was any is possibly not a good thing. Among others the possibilities raised are the following: first the resident immune cells of the brain are going to look at the construct as a pathogen and react accordingly. Second, if any brain cells start to express the protein, they will be attacked. Next, even if they are not, you've turned those cells into protein factories and there is the possibility of a biological cascade that starts with making a lot of spike protein, not all of which goes to the cell surface and winds up in a beta sheet confirmation. The mRNA is found in other organs as well, but likely is not as going to be always be a problem if it happens in organs that have some regenerative capacity.
If the assumption is that various nucleases will degrade the extracellular mRNA very rapidly and similarly the lipid formulation will be quite labile and hence the entire thing can't really go anywhere, this would appear to be untrue, unless the animal studies are just wrong.
Therefore, launching these vaccines without localization studies with brain function and histological analysis being done would appear to be an unusual risk.
How do the particles get into CNS? Given the work of the Gherardi group in Paris, it is possible they are picked up at the injection site by macrophages or other immune cells. This is only an hypothesis, however we know that macrophages do transport aluminum from the injection site into the CNS.