Phlorotannins are a class of polyphenolic compounds found exclusively in brown seaweeds (Phaeophyceae). There is emerging scientific evidence supporting their antiviral properties, though most of the research is preclinical and conducted in vitro or in animal models rather than humans. Several studies have demonstrated that phlorotannins can inhibit various viruses, including influenza A, herpes simplex virus, and some coronaviruses, by interfering with viral entry, replication, or protease activity. For example, phlorofucofuroeckol A from Ecklonia cava has shown inhibitory effects against influenza neuraminidase, and other phlorotannins have demonstrated activity against HIV and hepatitis C virus in cell-based assays.

The mechanisms underlying these effects appear to include direct interaction with viral proteins, antioxidant activity, and modulation of host immune responses. However, the majority of this evidence is limited to laboratory studies, and there is a lack of high-quality human clinical trials evaluating the efficacy of phlorotannins in treating or preventing viral infections. Thus, while the scientific rationale exists and preliminary results are promising, the current evidence level is moderate-to-low (rated 2/5) due to the paucity of human data.

In summary, phlorotannins have demonstrated antiviral potential in preclinical research, but more robust clinical studies are necessary to confirm their utility for viral infections in humans.