Bivalent influenza modRNA vaccine (BioNTech/Pfizer)

This study will be divided into two substudies - Substudy A (SSA) and Substudy B (SSB)
Substudy A This is a Phase 1 randomized substudy to evaluate the safety and immunogenicity of monovalent influenza modRNA vaccine (mIRV) and bivalent influenza modRNA vaccine (bIRV) at various dose levels, and quadrivalent influenza modRNA vaccine (qIRV), in participants 65 to 85 years of age. Participants will receive at Vaccination 1 either:
* 1 of 4 dose levels of mIRV (either A or B Strain),
* 1 of 4 dose levels of bIRV (containing both A and B strains),
* qIRV (at 1 dose level), or
* A licensed quadrivalent influenza vaccine (QIV).
At approximately 8 weeks following Vaccination 1, participants will be unblinded and QIV (Vaccination 2) administered to participants not having previously received this at Vaccination 1. Additionally, participants who previously received QIV at Vaccination 1 will receive one of the following for Vaccination 2:
* mIRV encoding A strain at dose level 4, or
* mIRV encoding B strain at dose level 4.
Substudy B
This is a randomized substudy to evaluate the safety and immunogenicity of the following vaccination schedules in participants 65 to 85 years of age:
2-Visit Schedules
* 2 doses of qIRV (at a dose level 1), administered 21 days apart.
* 2 doses of licensed QIV, administered 21 days apart (as a control group)
* A dose of licensed QIV following by a dose of bIRV encoding 2 A strains at dose level combination 1 or 2, administered 21 days apart.
1-Visit Schedules
* A dose of licensed QIV administered concurrently in the opposite arm with bIRV encoding 2 A strains at dose level combination 1 or 2.
* A dose of bIRV encoding 2 A strains administered concurrently in the opposite arm with a dose of bIRV encoding 2 B strains.at dose level 1.
* A dose of qIRV encoding 2 A strains and 2 B strains at dose level 2 (at one of two possible dose level combinations).
* A dose of qIRV encoding 2 A strains and 2 B strains at dose level 3.
* 1 dose of licensed QIV (as a control group).
Substudy B
In participants 18 to 64 years of age:
-A dose of qIRV encoding 2 A strains and 2 B strains at a dose level combination 1 or 2.