Coriolus versicolor (also known as Trametes versicolor or "Turkey Tail" mushroom) has a longstanding history in traditional East Asian medicine, primarily for immune support. In the context of influenza, its use is supported by some scientific evidence, though the data are limited and mainly preclinical or in vitro. Polysaccharopeptides extracted from Coriolus, such as PSK (polysaccharide-K) and PSP (polysaccharopeptide), have been shown in laboratory studies to modulate immune activity, increase cytokine production, and enhance the activity of natural killer (NK) cells, which are important in the defense against viral infections.

Some animal studies and cell-based assays suggest that these compounds might improve resistance to influenza virus infection by boosting innate and adaptive immune responses. For example, research published in International Immunopharmacology (2012) demonstrated that PSK could enhance T cell and NK cell responses in mice infected with influenza virus, leading to a reduction in viral titers and improved survival. However, robust clinical trials in humans specifically evaluating Coriolus for influenza prevention or treatment are lacking. Most human data pertain to its adjunctive use in cancer therapy, not infectious diseases.

In summary, while there is a scientific rationale and some preliminary evidence for Coriolus mushroom extracts supporting immune responses relevant to influenza, the direct clinical evidence for efficacy against influenza in humans is weak. Therefore, the evidence rating is 2/5, reflecting promising but limited support.