According to a recent study, an extract of black elderberry (Sambucus nigra L.) inhibited human influenza virus A (H1N1) infection in vitro. The IC50 value (i.e., the concentration at which the infection was inhibited by 50%) was 252 µg/ml. Two flavonoids isolated from the elderberry extract were found to bind to H1N1 virions and to block the ability of the virus to infect host cells: these flavonoids were 5,7,3′,4′-tetra-O-methylquercetin (Compound 1) and 5,7-dihydroxy-4-oxo-2-(3,4,5-trihydroxyphenyl)chroman-3-yl-3,4,5-trihydroxycyclohexanecarboxylate (Compound 2). Compound 1 and dihydromyricetin (the 3-hydroxyflavonone of Compound 2) were synthesized and shown to inhibit H1N1 infectivity in vitro by binding to H1N1 virions. Compound 1 had an IC50 of 0.13 µg/ml (0.36 µM) for H1N1 infection inhibition, while Compound 3 had an IC50 of 2.8 µg/ml (8.7 µM). The IC50 of the elderberry flavonoids compared favorably with those of oseltamivir (Tamiflu; 0.32 µM) and amantadine (27 µM), which are prescription drugs used to treat influenza.

The results of in vitro studies do not necessarily translate into clinical efficacy. In order for a compound to have an antiviral effect in vivo, it must be absorbed intact in amounts sufficient to achieve a virucidal concentration at the site of the infection. No data are available regarding the absorption of the antiviral flavonoids in elderberry, and the capacity of the human body to absorb other naturally occurring flavonoids has been found to be limited. However, in clinical trials, administration of an elderberry extract produced encouraging results in people with influenza-like illness.

Forty patients suffering from influenza-like symptoms during an outbreak of influenza B/Panama in 1993 were randomly assigned to receive, in double-blind fashion, a proprietary preparation (Sambucol) containing extracts of Sambucus nigra L. and raspberries (Rubus idaeus L.) or placebo. The dosage was 30 ml per day for children and 60 ml per day for adults, and the treatment was given for three days. Twenty-seven patients completed the trial. After two days of treatment, a higher proportion of patients in the active-treatment group than in the placebo group had experienced significant improvement in symptoms (93% vs. 25%; p value not stated). Symptoms resolved completely after three days in 87% of patients receiving active treatment and in 33% of those receiving placebo.

Sixty patients (aged 18-54 years; mean, 30 years) who were suffering from influenza-like symptoms for 48 hours or less were randomly assigned to receive, in double-blind fashion, Sambucol (15 ml 4 times per day during meals) or placebo for five days. The mean time until complete or almost-complete resolution of symptoms occurred was significantly less in the active-treatment group than in the placebo group (3.1 days vs. 7.1 days; 56.3% decrease; p < 0.001). No side effects were reported.

Further research with larger numbers of patients in whom the diagnosis of influenza is confirmed by laboratory studies is needed to confirm the results of these preliminary trials. The availability of natural substances should not deter people from taking appropriate precautions for preventing influenza (such as washing hands, keeping family members at home when they are experiencing influenza-like symptoms, and receiving vaccinations when clinically indicated). Nor should the availability of natural substances deter people from seeking medical care and taking medications such as Tamiflu when necessary.

¹ Roschek B Jr, Fink RC, McMichael MD, Li D, Alberte RS. Elderberry flavonoids bind to and prevent H1N1 infection in vitro. Phytochemistry 2009;70:1255-1261.
² Zakay-Rones Z, Varsano N, Zlotnik M, Manor O, Regev L, Schlesinger M, et al. Inhibition of several strains of influenza virus in vitro and reduction of symptoms by an elderberry extract (Sambucus nigra L.) during an outbreak of influenza B Panama. J Altern Complement Med 1995;1:361-369.
³ Zakay-Rones Z, Thom E, Wollan T, Wadstein J. Randomized study of the efficacy and safety of oral elderberry extract in the treatment of influenza A and B virus infections. J Int Med Res 2004;32:132-140.