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.
Participants in the study, however, reported only mild side effects that were consistent with any intramuscular vaccine injection, such as a sore arm or mild headache that might be experienced with a tetanus or a flu shot, said Dr. Robert Munford, a professor of internal medicine and a collaborator on the study.
Based on the protection they observed in the mice injected with ricin mixed with human-produced antibodies, the researchers projected that a RiVax-vaccinated human 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.
The results from the UT Southwestern trial justify further development of the vaccine, Dr. Vitetta said.
"We have shown that this vaccine is safe and immunogenic," said Dr. Vitetta. "Now we need to tinker with the dose and formulation to give the longest-lasting and most robust immunity."
Antibodies were present in the blood of study participants for as long as 127 days after the last vaccine injection, but the longevity of the antibodies in a given volunteer was not related to the dose level of the vaccine he or she received. 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. An adjuvant, such as alum, is an ingredient that enhances the immune response of a stand-alone vaccine. Other common vaccines, such as tetanus, are typically administered with an adjuvant and can confer immunity for years.