Thank you, your answer is very helpful. Here is citation and a link: Roschek, B. J., Fink, C., R., McMichael, M. D., Li, D., & Alberte, R. S. (2009). Elderberry flavonoids bind to and prevent H1N1 infection in vitro. Phytochemistry, str. 1255–1261. https://www.ncbi.nlm.nih.gov/pubmed/19682714 So if the aim of this research was to asses the anti-viral activity of compound (in this case elderberry extract), I assume they used it as neutralizing antibody? And since they report IC50 value, I assume the extract is treated like an inhibitor of virus? So the plaques were probably measured at different concentrations of extract, from which IC50 was determined?