The prophylactic efficacy of a low dose (100 mg) of
amantadine hydrochloride against experimental challenge with
influenza A/Texas/1/85 (H1N1) wild-type virus was determined in healthy adult volunteers in a placebo-controlled, double-blind, randomized trial. No side effects of the 100-mg dose were observed in the
amantadine-treated volunteers. Compared with placebo, 100 mg of
amantadine significantly reduced the frequency of illness (9 of 22 versus 2 of 22 volunteers, P less than 0.04) and provided 78% protection against
influenza illness. The two ill volunteers in the
amantadine group had
rhinitis only, whereas most of the ill placebo controls developed both systemic and upper-respiratory-tract illness. Wild-type virus was recovered from 50% of the
amantadine-treated volunteers, compared with 82% of the placebo controls. Of note, the infected
amantadine recipients shed 100 times less virus and shed virus for half as many days as did the infected placebo recipients. Although
amantadine restricted viral replication, it did not interfere with the development of an antibody response to influenza virus. These results indicate that in adults experimentally challenged with
influenza wild-type virus, 100 mg of
amantadine is effective both in the prevention of
influenza illness and in the restriction of virus replication.