Federal researchers have created a novel vaccine that not only sensitizes the body to an infectious agent but actually speeds up the body's immune system.

Using genetic engineering, Dr. Charles Flexner and others at the National Institute of Allergy and Infectious Diseases inserted the gene that produces human interleukin-2, a potent immune-stimulating compound, into the vaccinia, or cowpox, virus.

When the genetically altered virus was injected into so-called nude mice -- a strain that lacks most normal immunologic defenses -- the mice survived, the group reported in last week's issue of the British science journal Nature. Normally, a vaccinia inoculation kills nude mice.

"We have been looking for ways to reduce a virus's ability to cause disease without obliterating its use as a vaccine," Flexner said. "In the past, the means of making the virus less virulent made it less useful as a vaccine."

Australian researchers recently reported similar results with mouse interleukin-2.

Other members of the NIAID lab have been genetically altering a version of vaccinia to produce a prototype AIDS vaccine. If such a vaccine works -- and even that is unknown -- it might be possible to enhance it with the IL-2 technique.

Weakened live viruses have been used for years to prevent infections. Vaccinia was used to eliminate smallpox. But live virus vaccines also can cause diseases themselves if the virus regains any of its original characteristics. The polio vaccine, for example, occasionally reverts to a disease-causing form.

The problem is especially acute if an individual's immune system already doesn't work well, such as in individuals with genetic defects or who are receiving cancer chemotherapy. Earlier this year, physicians reported a vaccine-caused cowpox in an AIDS-infected military recruit.