Researchers at Northwestern University report progress in making a birth-control vaccine that would be effective yet reversible, with the potential for far fewer side effects than many currently available contraceptives.

A team headed by Dr. Erwin Goldberg is working on an injectable vaccine that simulates the action of a naturally occurring enzyme found on the surface of sperm. When injected into a woman, it can block conception by creating an immune reaction against sperm. The woman's body would then destroy sperm.

Experiments with female baboons "establish the principle" that the vaccine can be effective as well as reversible, Goldberg said in an interview. But he anticipates that it will be at least three years before the technology would be ready for human testing.

Experiments at the Evanston, Ill., campus involved a custom-made version of a sperm enzyme called lactate dehydrogenase C4. When female baboons were given injections every other month over a six-month period, three pregnancies occurred among 14 vaccinated animals, compared with 10 among 14 unvaccinated animals.

When the injections were stopped, nine of the 11 remaining vaccinated animals became pregnant, showing that the vaccine's effect wears off.

Goldberg said the contraceptive's effectiveness could be improved by changing the design of the vaccine and the dosage levels, as well as by adding chemical stimulators to boost the sensitivity of the immune system. "I would hope that we can achieve better than 95 percent effectiveness" in preventing pregnancy, he said.

Because the approach is not hormonal, Goldberg believes it will have fewer side effects than oral contraceptives or other birth control products. He said other laboratories are studying vaccines that affect the female egg or interfere with the growth of an egg once it is fertilized.