“What was important here was that this worked and was safe in animals similar to humans,” said Elaine Lissner, executive director of the Parsemus Foundation, the medical research organization behind Vasalgel that is also leading the trials.

A new study, published Tuesday, involved 16 male monkeys receiving injections of Vasalgel who were released into social outdoor housing with females. Any adverse effects, as well as their ability to impregnate females, was compared with a historical sample of monkeys that had received vasectomies.