https://www.stlzoo.org/animals/scienceresearch/reproductivemanagementcenter/contraceptionrecommendatio/contraceptionmethods/pzp-vaccine

PZP is not effective until after at least 2 injections (typically given at 2-4 week intervals). [This explains why they push 2 shots 4 weeks apart.]

Following the initial treatments, boosters are required, using only Freund’s Incomplete adjuvant. [This explains the boosters]

Longer-term use in animals not essential for breeding programs, because of the possibility of reversal failure following long-term use. [yip, sterilizes you with long term use]

https://pubmed.ncbi.nlm.nih.gov/12220157/

immunocontraception, All the PZP vaccine preparations tested were associated with a return to normal fertility within 1 year. [This explains the boosters after 6 months]