@KeiosKod From what i could understand, she tries to go back down to get another zombie blood sample, but probably fell unconscious while doing or something. So Yu went down to save her and got bitten. Sadly, She also got bitten so they both use the unfinished vaccine. The thing is, while her body were able to make the antibody, Yu couldn't. Or at least, not entirely since the virus (i will follow her guess) seems to still be able to affect the body. Either the white blood cell couldn't isolate and kill the virus entirely, or some other effect, i don't know. But i think the vaccine still works since it seems to stop the infection from affecting the brain too much. He technically becoming a zombie, with the craving and all, but with consciousness.