To summarize, quercetin, exhibits antiviral activity only when protected against oxidative degradation by ascorbate.

We compared the anti-poliovirus activities of three flavones, quercetin, luteolin and 3-methylquercetin, which differ only at ring position 3. 3-Methylquercetin was the most potent compound. Quercetin exhibited antiviral activity only when protected against oxidative degradation by ascorbate. The antiviral activity of luteolin was comparable to that of ascorbate-stabilized quercetin.

Protection of the antiviral activity of dietary flavonoids may contribute to the beneficial effects that have been claimed for vitamin C (Pauling, 1976).

Antiviral Activity of Flavones and Potentiation by Ascorbate
http://www.microbiologyresearch.org/docserver/fulltext/jgv/69/7/JV0690071749.pdf