Starting two weeks before flying, and continuing two weeks afterward, the travelers twice daily took a placebo or a combination of 600 mg of echinacea angustifolia root plus 675 mg of echinacea purpurea root and 4.4 mg of echinacea alkylamides. Participants doubled the dose while flying, and could triple or quadruple the dose for a short time when cold or flu symptoms occurred.
Researchers measured cold and flu symptoms before and immediately after travel, and again four weeks later. Upper respiratory symptoms increased for everyone during long-haul flights, but symptoms in the echinacea group were half as severe as symptoms were for placebo.