A Study of the BNT162b2 COVID-19 Vaccine Given at the Same Time as a Flu Vaccine in Adults.  A Phase 3, Randomized, Observer-Blind Trial to Evaluate the Safety and Immunogenicity of BNT162b2 When Coadministered With Seasonal Inactivated Influenza Vaccine (SIIV) in Adults 18 Through 64 Years of Age.