I want to know the answer to the following question:
What compound made the diphtheria vaccine safer to use?
All the background I'm provided with is that "The first diphtheria vaccines were produced in 1914 from a mixture of diphtheria toxin and antitoxin (produced from the serum of an inoculated animal), but the safety of the inoculation was marginal and it was not widely used.". Is that enough to answer the question?