The corrosion rate is not reversible, it can only be inhibited
The Cursed Children were constantly taking massive doses of corrosion-inhibiting medication to control the Gastrea inside their bodies, but it still only inhibited and did not stop the corrosion completely. Because the girls carried the inhibiting gene, they did not turn into Gastrea immediately like normal people, but if they suddenly used a lot of their power, or if Gastrea bodily fluids got inside them, then their corrosion rate would slowly increase. And then, like a normal person, if their corrosion rate went above fifty percent, then the corrosion would start, and they would not be able to keep their human form any longer. There was nothing that current medical technologies could do to change that limit.
Kanzaki, Shiden. Black Bullet, Vol. 1: Those Who Would Be Gods (pp. 211-212). Yen Press. Kindle Edition.
It is also the reason why Rentaro shot Kayo
Kayo Senju could not be saved. She had to die right here, right now.
Kanzaki, Shiden. Black Bullet, Vol. 1: Those Who Would Be Gods (p. 212). Yen Press. Kindle Edition.
In a conversation between Sumire and Rentaro it is also mentioned that the Cursed Children cannot be freed from the shackles that is their corrosion rate
Rentaro shook his head slowly. "That’s impossible, Doc. I can’t think of a life without Enju and Tina anymore." "But they will fall like leaves one day. Rentaro, don’t chase after falling leaves. The lifespan of Cursed Children is bound by the shackles of the corrosion rate of their body, which they cannot be freed from."
Kanzaki, Shiden. Black Bullet, Vol. 3 (light novel): The Destruction of the World by Fire (p. 81). Yen Press. Kindle Edition.