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

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 gener-ally accessible antiviral agents. Current treatments often face limitations such as early admin-istration requirements, resistance development, and limited global access. Natural products like European black elderberry (Sambucus nigra L.) fruit extract and quinine have been used histori-cally against viral infections. In this study, we investigated the antiviral efficacy of a standard-ized black elderberry fruit extract containing 3.2 % anthocyanins (EC 3.2) and, as a second natu-ral antiviral product, quinine, against IAV and SARS-CoV-2 in vitro. Madin-Darby Canine Kid-ney II (MDCKII) cells were infected with IAV PR-8, while human Calu-3 lung epithelial cells were infected with SARS-CoV-2 Wuhan type. Cells were treated with varying concentrations of EC 3.2 and quinine as either mono– or combinational therapy. Viral replication was assessed using quantitative RT-PCR, and cell viability was evaluated using WST-1 assays. Our results demonstrate for the first time that both EC 3.2 and quinine individually inhibited IAV replica-tion 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 anti-viral 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. These findings suggest that the combined use of black elderberry extract and quinine may serve as an effective antiviral strategy against IAV and SARS-CoV-2. The synergistic effect allows for lower doses of each product while keep-ing therapeutic efficacy. In summary, this combinational approach should be expanded to other respiratory RNA viruses, and thus might offer a promising avenue for pandemic preparedness. Moreover, the in vitro data obtained in this study have to be confirmed in clinical studies.