Flu Vaccine Army Spc. Selena M. Castillo prepares to administer a flu vaccination on Fort Sill, Okla., Nov. 15, 2019. Spc. Castillo is a medic for 3rd Battalion, 2nd Air Defense Artillery Regiment, 31st Air Defense Artillery Brigade. SHARE: Download: Full Size (1.95 MB) Tags: Vaccines, Army Credit: Army Sgt. Amanda Hunt VIRIN: 191115-A-WX120-002.JPG Photo Gallery