Numerous studies have showed blackcurrant extract to possess antiviral activity.

One in-vitro experiment showed that cells infected with influenza virus type A (IVA) or influenza virus type B (IVB) had their rate of infection almost stopped when blackcurrant extract was applied.

In another experiment, the addition of virus and extract simultaneously to cells resulted in infection rates 2 - 5% of that observed when only virus was added.

Mice fed extract exhibited reduced virus accumulation and improved mortality.

Fractionation of extract demonstrated antiviral activity is possessed by the polyphenol containing fraction. Polyphenols act on haemagglutinin, inhibiting a virus’s ability to enter infect a cell and reproduce within it. (Haemagglutinin is the molecule on the surface of viral particles that binds the virus to cells it is infecting.)

In a 2012 study Blackcurrants inhibit both viral adsorption onto cell surfaces and viral growth in cells. The antiviral effects of blackcurrants were observed across a wide spectrum of viruses, from RNA viruses (IFV and RSV) to DNA viruses (AdV and HSV). The follow-up 2013 study investigated the inhibitory effect of extracts of blackcurrant on four strains of influenza virus (IFV) by the inhibition of virus adhesion to the surface of the cells. 

The inhibitory effect of the extracts of blackcurrant or blueberry (the control) on the infectivity of the virus, were evaluated by the inhibition of virus adhesion on the cell surface.   

The study showed that although the antiviral effect of blackcurrant was slightly different within viral strains from one species, the extract of blackcurrant could disinfect all of four influenza strains that were examined.