First tablet for smallpox

The smallpox virus was the target of an eradication programme

Scientists say they are well on the way to developing the first oral drug to tackle smallpox infection in humans.

Although a smallpox vaccine has been used successfully throughout the world to prevent the disease, there are no antiviral treatments which have proved effective against the infection.

There have been fears that the virus could be used as a biological weapon, as few people in Western countries are vaccinated.

One drug, cidofovir, is promising, but has to be given through a vein, limiting potential to use it in a mass outbreak of the disease.

Cidofovir appears able to limit the replication of the virus if given shortly after infection.

Researchers from the University of California San Diego and the Veterans Affairs San Diego Healthcare System say they have found a derivative of cidofovir - HDP-CDV, which is far more powerful, yet might be suitable to be given in tablet form.

More powerful

In laboratory tests, HDP-CDV appeared to be 100 times more potent than the original drug, and better able to get inside cells to get access to the virus.

However, as these tests have only been carried out on human tissue cultures in the test tube, and in mice infected with cowpox - a similar infection.

Extensive safety testing will be needed before it can be declared suitable for humans.

However, the research team, announcing its findings at the International Conference on Antiviral Research in Prague, said that hopes were high.

Vaccine stocks

Karl Hostetler, one of the scientists leading the research, said: "If you've got thousands of people exposed to smallpox, a drug that needs to be injected would be difficult to use widely.

"Until now, the eradication and control of smallpox relied upon vaccination.

"The results suggest that antiviral drugs given orally in a regimen consisting of as few as five doses might be used as an alternative to treat and contain a future outbreak of smallpox, especially in those individuals who cannot safely be vaccinated."

The events of September 11 have again heightened fears of biological attack, and the US government has announced plans to massively increase its stocks of smallpox vaccine.

However, some patients do suffer damaging reactions to vaccination, so routine immunisation is not recommended.