For those who did not read the books and did not watch yet the film, it may spoil the ending.

In the epilogue of book 3, Ava Paige writes "I anticipated this outcome and put into place an alternate solution, to save at least a portion of our race. With the help of my partners, two wisely placed Immunes, I was able to plan and implement a solution that will result in the best outcome we could’ve hoped for."

Brenda's words to Thomas in their last paragraph "there are over 200 of us, and we're all immune. It'll be a good start" - imply she is one of the aforementioned "partners", but who is the second?