I have thought about this quite a lot over these many years. I have a whole theory:
Basically, My thought is that the secret to the cure's recipe was that the blood base was the placenta (afterbirth) of one of the royal females who had the rare blood type AO-, like Coraline and Beth.
Maybe that would explain one reason why Coraline was one of so many siblings.
Coraline's mother probably also had the royal blood type.
Could Beth actually be a descendent of the royal family? I think she might be. (Sorry off topic)
The reason it was a secret was because the Royal physician who invented the cure never told the King or any of the royal family. He did not want to relinquish control to the King. He and the King were not always on good terms. Finally, he destroyed the laboratory, and supposedly everything need to make more of the cure. He and the Royal Gardner even burned the fields were the flowers and plants grow that were used to make the cure.
His research notebooks however were sent to America, years before the explosion for safe keeping.
...
Sorry to much to tell here.
Look forward to reading other theories.