Thank you for continuing the thread. A lot of vaccines do contain aluminum adjuvants, but Merck has an apparently proprietary adjuvant that sounds like a nano-particle to me. It is called amorphous aluminum hydroxyphosphate sulfate. I don't think it's exact composition could be known outside of Merck as it is, as I said apparently "proprietary".
https://pubmed.ncbi.nlm.nih.gov/17581283/
I would think this adjuvant (AAHS) would be very susceptible to being taken up by macrophages and possibly transported to the brain.