The antiviral property of vitamin C may be due to the fact that it inhibits the viral enzyme neuraminidase. - GreenMedInfo Summary

Several pathogens, both viral and bacterial, employ the enzyme neuraminidase (N-acetylneuraminate glycohydrolase, EC 3.2.1.1.8). The neuraminidase renders ineffective the hemagglutinin inhibitory mucins that confine the pathogens in a coating of host mucins. Sialoresponsin is a receptor "decoy" that inhibits neuraminidase. Several known antiviral agents, including ascorbic acid, inhibit neuraminidase. It is proposed that ascorbic acid may mediate an antiviral effect through the incorporation of ascorbic acid or some derivative of ascorbic acid as a part of the sialoresponsin molecule. Whether ascorbic acid works alone as a pharmacological inhibitor, or is incorporated in sialoresponsin as a physiological inhibitor; it may be useful against pathogens that employ neuraminidase.