“To provide a vaccine with an improved stability profile, the Pfizer-BioNTech COVID-19 Vaccine for use in children 5-11 years of age uses tromethamine (Tris) buffer instead of the phosphatebuffered saline (PBS) as used in the previous formulation and excludes sodium chloride and potassium chloride.”

https://citizenfreepress.com/breaking/pfizer-adds-ingredient-to-vaccine-for-kids-used-to-stabilize-heart-attacks/