Gabe in the After / Shannon Doleski.

Enhanced descriptions from Syndetics:

It's fourteen-year-old Gabe Sweeney's day to check for survivors . . .

Two years after a global pandemic, twenty survivors (most of them children) have relocated from their coastal Maine island full of sad memories to a mansion on a small, neighboring island where they have school and farm chores. When Gabe and his dog, Mud, find Relle Douglas alone in the woods on the mainland, they take the strange new girl across the channel to live with them.
Relle changes the island with her hopeful attitude. She tells big stories and makes plans for activities like talent shows. Despite a growing crush, Gabe doesn't quite understand the point of it all; why have a talent show at the end of the world? But when tragedy strikes, Gabe sets out on a dangerous journey to try and find other survivors where the world might be normal. Like Before.

Lightly inspired by Anne of Green Gables , Gabe in the After is a moving and heartfelt story about the end of the world--and what perseveres through it.