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It is universally accepted that there will be another flu pandemic, and despite advances in medicine there will probably be many fatalities - as many as 50 million as in 1918, or 'only' a million or so as in 1957 or 1968. The new flu will be derived from one of the 144 type A bird influenza viruses that peacefully coexist in nature with their waterfowl hosts. Today, flu is controlled by vaccines, anti-viral drugs, and infection limitation, but there are problems: a new vaccine could take many months to make, virus is already resistant to the main drug (Tamiflu), and it may be impracticable to isolate people for the duration of a pandemic. Into this picture comes a new concept in antivirals - protecting virus.

Protecting viruses are natural virus variants that have lost a substantial chunk of their genome through a faulty replication event and cannot multiply. Protecting virus is given intranasally and acts instantly. It remains quiescent in cells until it is joined by a normal virus. Normal virus replicates both its own genome and that of the protecting virus, but as the protecting virus genome is 5 times smaller, 5 copies are made in the time it takes to make one copy of the full-length genome. As a result, the majority of new virus particles formed incorporate protecting RNA, and these go on to protect neighbouring cells. Infection aborts without clinical disease, and virus is cleared by host defences. Because protecting influenza virus acts at the level of genome replication it can act on any strain of flu A. Next up are human trials.