The study will assess and compare the effects and efficacy of these two immunocontraceptive vaccines.
The first vaccine, GonaCon, was developed in the 1990s in the United States by the National Wildlife Research Center, a research unit of the U.S. Department of Agriculture (USDA) whose mission includes developing solutions to problems caused by wildlife. GonaCon has since been used in numerous wild and domestic animal species, including bison, horses, nutria, feral cats, domestic pigs, deer, and wild boar. Its mode of action is based on the neutralization of Gonadotropin-Releasing Hormone (GnRH), the hormone that regulates reproduction in all mammals. The vaccine consists of this hormone linked to an immunogenic carrier protein (derived from a mollusc), together with an adjuvant that enhances the immune response. Administration of the modified synthetic GnRH stimulates the animal's immune system to produce antibodies that neutralize the naturally occurring GnRH in the body, thereby rendering the animal temporarily infertile. The contraceptive effect of GonaCon is reversible, and its duration of effectiveness—ranging from one to six years, or even longer—depends on the animal species concerned.
A new formulation (GonaCon B) has subsequently been developed. It provides a longer-lasting immune response following a single injection, although it is still only sparsely documented.
The second vaccine, SpayVac, was developed in the 1990s by the Canadian company ImmunoVaccine Technologies, whose assets were subsequently acquired and commercialized by BioVaxys Technology Corp. Commercial rights for wildlife applications are licensed to SpayVac for Wildlife.
SpayVac is based on porcine zona pellucida (PZP) antigens encapsulated in liposomes. This technology substantially reduces the side effects associated with conventional PZP vaccines and, through the slow release of the antigen, provides prolonged immunization. Its use by Thailand to reduce elephant populations has recently attracted considerable media attention.
Following the promising results obtained in 47 red deer vaccinated with a single dose in a Bavarian forest in Germany—reducing the pregnancy rate of vaccinated females to 11%—SpayVac for Wildlife initiated, in 2025, the regulatory approval process for the use of the vaccine in deer and horses in the United States. Similar approval procedures are also underway in Australia.