Vaccine that has not been mixed has specific beyond-use dates for frozen and refrigerated storage. Use these labels to ensure beyond-use dates are followed. Storing Vaccine in the Freezer Pfizer-BioNTech COVID-19 Vaccine may be stored in the freezer between -25°C and -15°C (-13°F and 5°F) for up . to 2 weeks.
https://www.cdc.gov/vaccines/covid-19/info-by-product/pfizer/downloads/bud-tracking-labels.pdf

