Thank you for raising that question. Looking back at the biodistribution studies, the EMA report stated that 2-4% of Moderna's mRNA vaccine in the plasma (blood) crossed the blood-brain barrier, but these were cleared after ~25 hours (as shown in the image in this Medium article). The Moderna EMA report did not provide specific percentages for other organs.
For Pfizer's mRNA vaccine, the Japanese study found that <0.1% entered the brain and other organs (e.g., adrenal glands and ovaries) at 48-hour, which is consistent with the EMA report about Pfizer's mRNA vaccine.
So, it seems that less of Pfizer's vaccine ended up in other tissues compared to Moderna. At least for the brain, Moderna's vaccine becomes undetectable after ~25 hours. I can't seem to find data on how long Pfizer's mRNA vaccine persist in the organs, but if I may guess, it would be after a few days given that only <0.1% were detectable at 48-hour.
All that said, the higher dose in Moderna's mRNA vaccine may be an advantage as it could mount stronger immune reactions, providing higher protection against the Delta variant, although this is just a preliminary evidence for now:
