Another possibility is to use a modified organism specific only to possums, which carries an extra gene to produce the specific sperm or egg protein. When the organism infects the possum the extra gene would cause it to produce copies of the sperm or egg proteins. The possum's immune system would then produce antibodies against this foreign protein, sperm and eggs would be attacked in the reproductive tract and the ovaries, and infected female possums would become infertile. A sperm-based contraceptive vaccine could also make male possums infertile. The vaccine makes the possum infertile by tricking its immune system into treating the sperm or egg as a foreign body.
Antibodies attack the sperm and eggs and prevent fertilisation.
Possums sterilised by this modified organism could eventually regain their fertility as the immune reaction declined, but possums would be susceptible to reinfection and consequent reimmunisation. The contraceptive effect could be longer lasting in females as each new mating would act as a booster vaccination with a new dose of sperm or egg proteins. Ideally the organism chosen to transmit the vaccine would be humane, possum specific and spread by close contact between possums. The completed contraceptive vaccine may take 10 to 15 years to develop as it is essential that such a vaccine will only affect possums. A bait delivery system may be available within five to seven years.