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Effectiveness of an herbal preparation containing echinacea, propolis, and vitamin C in preventing respiratory tract infections in children. A randomized, double-blind, placebo-controlled, multicenter study
H.A. Cohen, I. Varsano, E. Kahan, E.M. Sarrell, Y. Uziel
Archives of Pediatrics and Adolescent Medicine, 2004; 158: 217-221