The U.S. government, since the terrorist attacks on Sept. 11, 2001, and the anthrax letters delivered shortly afterwards, has been rushing to develop a safer smallpox vaccine to help prepare for the threat of an intentional release of the deadly virus.

The only currently licensed smallpox vaccine in the United States - DryVax, made by Wyeth in Marietta, Pa. - can have life-threatening side effects.

In the study, which appears in the September issue of the Journal of Virology, researchers at the National Institute of Infectious Diseases and the National Cancer Center Research Institute in Japan inoculated mice with VaxGen's candidate vaccine LC16m8, which is a modified strain of the vaccinia virus that is intended to have fewer side effects than DryVax.

The results indicated a single shot of the vaccine completely protects mice from vaccinia, said VaxGen, based in Brisbane, Calif.

The researchers also compared LC16m8 to the Lister strain of vaccinia, which was used in smallpox vaccines that successfully eradicated the disease worldwide. The strains are very similar on a genetic level, leading the researchers to speculate that LC16m8 would be as effective in protecting against smallpox, VaxGen said.

Smallpox kills about 30 percent of those it infects, and it can also leave severe, disfiguring scars, according to the Centers for Disease Control and Prevention.

The National Institutes of Health has funded studies of another smallpox vaccine called modified vaccinia virus Ankara or MVA. This vaccine is manufactured by Therion Biologics of Cambridge, Mass., and also contains a weakened or attenuated strain of vaccinia virus with the aim of reducing the potential for life-threatening side effects.

Dr. Anthony Fauci, director of the NIH's National Institute of Allergy and Infectious Diseases, testified before Congress in July that MVA looked promising in animals and early clinical trials in people.

Fauci told United Press International MVA is moving its way toward being a candidate for Project Bioshield, but the government has not yet signed a contract to acquisition supplies of the vaccine.

Fauci said LC16m8 is analogous to MVA in the sense that attenuated smallpox vaccines seem to be safe. "The thing we haven't been able to demonstrate conclusively is whether (the attenuated vaccines) are effective" in protecting people from contracting smallpox, he said.

The current study with mice that received LC16m8 and were protected from a challenge with vaccinia virus is a good indication that these vaccines will be protective in people, but the Food and Drug Administration may require further studies before granting approval, he said.

VaxGen is developing LC16m8 in the United States. The vaccine, which is manufactured by the Chemo-Sero-Therapeutic Research Institute in Kumamoto, Japan, is in Japan's emergency stockpile and has been studied in 50,000 Japanese children.

VaxGen spokesman Paul Laland told UPI the company has just completed a phase I/II trial of LC16m8 involving 163 healthy volunteers, and so far the vaccine seems to have an acceptable safety profile.

The results of the trial have not yet been released, but Laland said the company plans to present it at a scientific meeting sometime this year.

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