This is a very tough decision. Here, choosing between humanity and Clementine, I would chose humanity, as it's the most rational way to go. As much as I like Clementine, I can't screw the whole humanity just because of one life - these are my thoughts in that situation.
But I also tried to look at this from a different perspective. If I take Clementine out of this and replace her with someone I really love, the situation becomes ten times harder, and my previous "rationalized" choice won't happen, because I know that I would not be able to sacrifice that person.
I still stay with my decision to safe vaccine over Clementine, because again, despite that I like her, she's just a fictional character and thus I am able to rationally think through this situation without being overwhelmed with my emotions completely - no matter how well-written and played out the character is, it can't trick my mind completely.
But if I were in the similar situation with that person mentioned earlier, I know I'd lose the vaccine.
The aftermath... I would be in a great depression after saving the vaccine, and I would blame myself till the end of my life. But at the same time, if the vaccine actually works and saves the humanity... I would keep asking myself till the end: "You've killed a kid to safe millions. Was it worth it?.."
ˇPrepararse para los problemas!
ˇY mas vale que teman!