My guess for now is Sunny may not be a researcher but some sort of spy and that lab she claimed she's from was actually a place she infiltrated into to stole the vaccine and maybe caused a security breach with ended up making that place overran by zombie, heck, maybe that was the place that researched both virus and vaccine and she was the reason of the outbreak in the first place.
That's maybe the reason she seems to carry some heavy guilty conscience and feel like she has keep going on this mission of her.