I think the ending is really interesting. It's not a happy ending but it also doesn't give the catharsis of a total disaster. Things continue to get worse and the elements of hope that were there are not fully dashed, but muddied and left ambiguous.
Was Watts actually onto something with the cure or are the other scientists now just chasing bullshit? There's no clear answer on if he lost a genuine cure or if he was just deluding himself or even infected and succumbing to the effects of Trixie.
David thinks that he's immune but towards to end has started to get a lot more comfortable with killing and doesn't even care enough to tell the doctors to test him for immunity when they explicitly bring it up. Is that also a sign of infection or just him being deeply traumatised? If he is immune they're not testing him anyway and he's not saying anything.
Will Peckem be of any use stopping the spread in Louisville or is he just going to be another vector when they pull him out of Evans?
Things are unravelling, the systems trying to keep things under control are falling apart, it doesn't look good, but we never get that big collapse, just Peckem's view as he gets airlifted of the small window of light of the landing sight getting consumed by the night.