There's no (((reputable))) link, but here are some sources:
From page 14 of the FDA Briefing Document (PDF warning) (fda.gov):
the Pfizer-BioNTech COVID-19 Vaccine for use in children 5-11 years of age uses tromethamine (Tris) buffer instead of the phosphate-buffered saline (PBS)
From a description (go.drugbank.com) of tromethamine:
Tromethamine is a proton acceptor used for the prevention and correction of metabolic acidosis associated with various clinical conditions, such as cardiac bypass surgery.
There's no (((reputable))) link, but here are some sources:
From page 14 of the [FDA Briefing Document (PDF warning)](https://www.fda.gov/media/153447/download):
> the Pfizer-BioNTech COVID-19 Vaccine for use in children 5-11 years of age uses tromethamine (Tris) buffer instead of the phosphate-buffered saline (PBS)
From a [description](https://go.drugbank.com/drugs/DB03754) of tromethamine:
> Tromethamine is a proton acceptor used for the prevention and correction of metabolic acidosis associated with various clinical conditions, such as cardiac bypass surgery.
Archive of PDF: https://web.archive.org/web/20211031023136/https://www.fda.gov/media/153447/download
(post is archived)