Immunocompromised patients age 6 months or older who are given mRNA vaccine are recommended to receive an age-appropriate 3-dose primary series. Both Moderna and Pfizer-BioNTech products have different doses beginning at age 12 years. What dose do I use for a severely immunocompromised patient that starts the series at age 11 years but turns 12 before completing the 3-dose primary series?

CDC states that if a person moves to an older age group between vaccine doses, they should receive the vaccine product and dosage for the older age group for all subsequent doses. 

Children who transition from age 11 years to age 12 years during the initial vaccination series should complete the 3-dose series using the dosage for people ages 12 years and older for all doses received on or after turning age 12 years: 

Moderna series: 2024–2025 Moderna, 0.5 mL/50ug 

Pfizer-BioNTech series: 2024­–2025 Pfizer-BioNTech, 0.3 mL/30 ug 

See www.cdc.gov/vaccines/covid-19/clinical-considerations/interim-considerations-us.html#table-02 for additional details.