The Pairings - The Pairings, #1

The secret is locked in her mind-and they'll do anything to get it. Generations ago, humanity was devastated by a virus that wiped billions from the planet. If not for AmeriGlobe's Pairing programme to match immune survivors, and VaxWell's immunity-boosting biomods, humans would've died out.  Lora Flannigan has sought to conform to the strict laws throughout her eighteen years, and after the Testing she now stands on the brink of a prestigious pairing with high-caste Jarid.
Accepting the match will pull her and her family up from their low-caste existence.  But Lora never counted on the sparks that ignite when she meets her Pair's striking brother: Syeth-dark, brooding and harbouring secrets. But before she's managed to make sense of her feelings, he's embroiled her in the rebellion underworld.  And when still darker truths come to light, Lora realizes the purpose behind not only her pairing.
but her very existence.