Polyriboinosinic-polyribocytidylic acid [poly(IC ⅐ LC)] was evaluated for its prophylactic and therapeutic efficacies against respiratory influenza A virus infection in mice. Two doses of poly(IC ⅐ LC) (1 mg/kg of body weight per dose) administered intranasally within 12 days prior to infection with 10 50% lethal doses of mouse-adapted influenza A/PR/8 virus fully protected the mice against the infection. Determination of virus titers by hemagglutination and plaque assays showed more than a 2-log 10 decrease in virus titers in lung homogenates of pretreated mice compared with those in the lungs of the nonpretreated group. Treatment of infected mice with poly(IC ⅐ LC) resulted in a modest (40%) survival rate. These results suggest that poly(IC ⅐ LC) provides a highly effective prophylaxis against respiratory influenza A virus infection in mice.

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