An experimental contraceptive vaccine has blocked fertility without fail in female and male guinea pigs, researchers report, raising prospects that a similar approach might one day work for women and men.

The vaccine is designed to prevent fertilization, which may make it more widely acceptable than another vaccine already in human testing that stops development of the embryo, other scientists said.Still, "there are many things about it that would have to be changed or improved to make it a useful method for either agricultural animals or humans," cautioned researcher Paul Primakoff.

Primakoff and colleagues at the University of Connecticut Health Center in Farmington report the experiment in Thursday's issue of the British journal Nature.

It is the first demonstration of contraception without fail from a vaccine, experts said.

None of the 25 female guinea pigs that got the vaccine before mating had litters, nor did the mates of the six immunized male guinea pigs. Animals that received sham immunizations for comparison purposes remained fertile.