For this randomised , comparator-controlled trial , we randomly assigned ( 1:1 ) healthy adults ( aged 18-64 years ) who attended one of four employee health clinics in the University of Colorado health system , with stratification by site , to receive one dose of the trivalent inactivated influenza vaccine Afluria given either intramuscularly with a needle-free jet injector ( Stratis ; PharmaJet , Golden , CO , USA ) or with needle and syringe .