Two new agents, which can knock out the flu virus once it has taken hold and stop symptoms, have been tested in Europe and the US and new trials involving more than 1,000 patients are beginning in the UK and neighbouring countries this winter.
The drugs, called neuraminidase inhibitors, have a unique action which does not stop infection, but prevents the virus spreading within the body. One of the drugs, zanamivir, has been tested on 2,000 patients in 15 countries and results are said to be "promising". They are to be published in the New England Journal of Medicine shortly.
A race is now on between the multinational firms Glaxo-Wellcome, developers of zanamivir, and Hoffman La Roche, which has a rival agent known as GS 4104, to be the first to win approval for the drugs, which could have a market worth billions of pounds worldwide.
If approval is granted, they could be available next year. The companies hope eventually to sell them over the counter like cough and cold remedies.
The only current defence against flu is vaccination, which is ineffective once infection with the virus has occurred.
Dr Douglas Fleming, head of the Royal College of General Practitioners' flu monitoring unit in Birmingham, said: "The concept of treating patients on this basis [after infection] is a breakthrough. It is quite novel."
Professor John Oxford, head of virology at the Royal London Hospitals NHS Trust, who is conducting trials on GS 4104, said: "These compounds are a tremendous breakthrough. I am very optimistic.
"Everyone is going along with the view that they will hit the jackpot."
The drugs work by blocking the action of neuraminidase, an enzyme "spike" on the surface of the flu virus which enables it to migrate through mucus in the lungs and spread among cells.
Dr Fleming said: "It is like fitting a glove over the spike which inhibits its effect. You can stop the virus in its tracks if you put the gloves on." However, timing was "critical", he added.
If given too late in the course of the illness, the drug had little effect. Since flu symptoms normally take a couple of days to appear, their value may be limited.
Professor Oxford said that GS 4104 had been shown to prevent infection in volunteers given it before exposure to the virus. Trials were now being carried out to test its therapeutic effect after infection.Reuse content