The perfect medium for sterilization, birth control after 1 child, or other nefarious purpose. Imagine if vaccinations, including flu shots, are compulsory in all 50 states (like California). They can give us Proles anything they want...
If a one-shot sterilization vaccine was out there, I think we would know. Many, many people would love to be permanently, nonsurgically sterilized after their one or two children.