Pfizer changed their formulation for 5-11 year old kids. They added a blood acid reducer which is used to stabilize people with heart attacks.

Page 14 of this PDF.

www.fda.gov/media/153447/download

Vaccine formulation
Authorization is being requested for a modified formulation of the Pfizer‑BioNTech COVID-19 Vaccine. Each dose of this formulation contains 10 μg of a nucleoside-modified messenger RNA (mRNA) encoding the viral spike (S) glycoprotein of SARS-CoV-2 that is formulated in lipid particles and supplied as a frozen suspension in multiple dose vials.

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. The packaged vials for the new formulation are stored frozen at -90°C to – 60°C. The frozen vials may be thawed and stored at refrigerator at 2°C to 8°C for up to 10
weeks.

The Pfizer‑BioNTech COVID-19 Vaccine does not contain preservative. The vial stoppers are
not made with natural rubber latex. For the 10-μg RNA dose, each 1.3-mL filled via vial must be
diluted with 1.3mL 0.9% sodium chloride for injection to provide 10 doses at 10 μg RNA / 0.2 mL
Injection volume. After dilution, the vials should be stored at 2°C to 25°C and should be used
within 12 hours

Tromethamine (Tris)

www.google.com/search?kgmid=/m/02hd7f&hl=en-US&q=Tromethamine&kgs=179ea66b1daf6c7d&shndl=0&source=sh/x/kp/1&entrypoint=sh/x/kp

h/t New Normal Blows