Synergistic Antiviral Activity of European Black Elderberry Fruit Extract and Quinine Against SARS-CoV-2 and Influenza A Virusa

Background/Objectives: The persistent threat of emerging respiratory RNA viruses like SARS-CoV-2 and Influenza A virus (IAV) necessitates the continuous development of effective, safe, broadly acting, and generally accessible antiviral agents. Current treatments often face limitations such as early administration requirements, resistance development, and limited global access. Natural products, like European black elderberry (Sambucus nigra L.; S. nigra) fruit extract and quinine, have been used historically against viral infections. In this study, we investigated the antiviral efficacy of a standardized black elderberry fruit extract containing 3.2% anthocyanins (EC 3.2) and, as a second natural antiviral product, quinine, against IAV and SARS-CoV-2 in vitro. Methods: Madin–Darby Canine Kidney II (MDCKII) cells were infected with IAV PR-8, while human Calu-3 lung epithelial cells were infected with Wuhan-type SARS-CoV-2. Cells were treated with varying concentrations of EC 3.2 and quinine either as mono- or combinational therapy. Viral replication was assessed using quantitative RT-PCR, and cell viability was evaluated using WST-1 assays. Results: Our results demonstrate, for the first time, that both EC 3.2 and quinine individually inhibited IAV replication in a dose-dependent manner, with IC50 values of approximately 1:400 for EC 3.2 and 250 nM for quinine. Most importantly, the combinational treatment exhibited a strong synergistic antiviral effect, as confirmed by the Bliss independence model (synergy scores of 14.7 for IAV, and 27.8 for SARS-CoV-2), without affecting cell viability. Conclusions: These findings suggest that the combined use of black elderberry extract and quinine might serve as an effective antiviral strategy against IAV and SARS-CoV-2, particularly since the synergistic effect allows for lower doses of each product while retaining therapeutic efficacy. In summary, this combinational in vitro approach, when expanded to other respiratory RNA viruses and confirmed in clinical studies, has the potential to open a promising avenue for pandemic preparedness.