'Protecting virus' has a significant alteration to one of the virus's genes. The genetic material of a flu virus consists of 8 individual segments of single stranded RNA. Professor Dimmock's protecting influenza virus has a huge but specific deletion of around 80% of the RNA of one of these 8 strands.
This deletion makes the virus harmless and prevents it from reproducing by itself within a cell, so that it cannot spread like a normal influenza virus. However, if it is joined in the cell by another influenza virus, it retains its harmless nature but starts to reproduce - and at a much faster rate than the new influenza virus. This fast reproduction rate - spurred by the new flu infection - means that the new invading influenza is effectively crowded out by the 'protecting virus'. This vastly slows the progress of the new infection, prevents flu symptoms, and gives the body time to develop an immune response to the harmful new invader. In effect the protecting virus converts the virulent virus into a harmless live vaccine.
See the University of Warwick for more info. This approach might become commercially viable pretty quickly, although testing in humans hasn't happened. The risk would be for harmful mutation of this counter virus somehow, and that risk can only be assessed by careful experimentation. However they say the good Prof has been working on this for 20 years, and that the first phases of animal testing have been done. The patent's been granted and the university has formed a company:
The Warwick research team has now filed a patent on the protecting virus and they are exploring ways of taking 'protecting virus' through human clinical trials and testing on birds. The University has established a company - ViraBiotech - to help advance those aims. This may involve venture capital support, and collaborations with pharmaceutical companies, to enable this novel technology to be rigorously tested in a wide range of animals and humans, and using a wide range of influenza strains.
There is another story out about a peptide-based blocker of flu virii, but it will take some time before it could possibly be brought to widespread use, if it is workable at all. It's very promising, but preliminary.