Melanie and Justinaeu's relationship wasn't fleshed out enough

The book made a maternal connection between them, the way Melanie was drawn to Justineau's black skin and curly hair. This maternal connection would explain why they are so close, why Justinaeu would protect Melanie even to the detriment of a vaccine development.

But this movie didn't convey that detail.

First - I think they made a mistake by not casting a black woman in Justineau's role. This prevented any sort of mother - daughter thoughts.

The viewer is supposed to believe that Justineau protected Melanie so strongly just because Justineau was really nice. It wasn't convincing. In reality, a person would sacrifice Melanie to save humanity.