Protective efficacy of low-dose amantadine in adults challenged with wild-type influenza A virus

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.