Scientists at the University of Texas Southwestern Medical Center at Dallas, say a vaccine against the deadly toxin ricin is safe and effective.

Researchers have completed the first human clinical trial of a the vaccine against ricin which is a potential bioterror threat.

The long pilot study involved three groups of five volunteers each.

Individuals in each group received a series of three injections of various doses of the vaccine, called RiVax, over a period of just under one year.

The five individuals in the group receiving the highest vaccine dose produced ricin-neutralizing antibodies in their blood, indicating their immune systems had responded,while four of five in the intermediate dose group produced antibodies.

In the lowest dose group one of five did so.

The human-produced antibodies were then injected along with active ricin toxin into test mice, and the mice survived.

Dr. Ellen Vitetta, director of the Cancer Immunobiology Center at UT Southwestern Medical Center and lead author of the study, says their main concern was safety as they were dealing with a deadly toxin.

The study has received international attention as the experimental vaccine grew out of the team’s cancer-therapy work.

Ricin, which can be administered in food and water or sprayed as an aerosol, is extracted from castor beans.

There is currently no approved vaccine to prevent ricin poisoning in humans, and the biological agent has a long history of use in espionage.

Only mild side effects were reported by the participants such as a sore arm or mild headache, that might be experienced with a tetanus or a flu shot.

Based on the trial the researchers believe that vaccinated humans could withstand a lethal dose of injected ricin.

According to the Centers for Disease Control and Prevention, such a dose for an adult is as little as approximately 500 micrograms of ricin, an amount that would fit on the head of a pin.

Dr. Vitetta and her colleagues are now conducting studies on mice that combine the vaccine with an adjuvant, a formulation that may lengthen the time the vaccine is effective.

Depending on how the ricin is administered as a poison, victims develop fever, nausea and abdominal pain or lung damage before dying within a few days of exposure.

At present there is no antidote after the first few hours of exposure and, because symptoms do not appear until later and often mimic other illnesses, individuals often do not know if they have been exposed until it is too late for treatment, Dr. Vitetta said.

The results of the vaccine trial will be published in the next issue of the Proceedings of the National Academy of Sciences.