"(...) several types of drugs are known to diminish male fertility, but those that have been tested have serious problems of toxicity. Very little work is in progress on immunological methods, such as vaccines, to reduce fertility, and much more research is required if a solution is to be found here."
The possibility of using vaccines to reduce male fertility was something that needed to be investigated further, according to the Rockefeller Foundation, because both the oral pill and the IUD were not suitable for mass-scale distribution:
"We are faced with the danger that within a few years these two "modern" methods, for which such high hopes have been held, will in fact turn out to be impracticable on a mass scale."
A d v e r t i s e m e n t
The possibility of administrating hormone preparations to reduce fertility was also mentioned, although- states the report- they have been known to "cause bleeding problems, which may limit their usefulness."
"A semipermanent or renewable subcutaneous implant of these hormones has been suggested, but whether or not the same difficulties would result has not been determined." Source