I hadn’t actually heard of this film until recently and I think the two reasons for this were as follows:
1. As a subgenre, apocalyptic Zombie films have literally been done to death, to the point where if you say ‘fancy a zombie film?’ 2/3 of people will wince and say ‘not another one’. At which point you have to convince the other party ‘no, but there’s a twist, it’s not like the others’; this is always a hard sell.
2. Considering the talent involved and the allegorical themes it sets itself, I think the filmmakers were trying to downplay the ‘zombie/horror’ theme and market it as  a serious film for the arthouse crowd. Again, you say the word zombie to someone over the age of 25 and you’ve probably lost half your audience already, or perhaps it’s only people over 25 who still enjoy zombie films. I’ve met a few who think horror films are stupid and immature.
Without a doubt this is a zombie horror film. The critical difference here, and one which distinguishes it from other horror films, is that like The Walking Dead and Romero’s original ‘Living Dead’ films, is that the drama, the characters and symbolism of the zombies have a strong  moral message. Not a very interesting message, but one nonetheless. Basically ‘humans are bad, they’ve gotta go: get used to it’.
Despite how this sounds, interestingly the humans, mostly the men, aren’t really that bad at all. Normally there’s one or two characters who seem particularly evil and sadistic, and although the way the soldiers and scientists treat their child test subjects is coarse and unsympathetic they do so for a very good reason, and herein lies the central premise of the film.
But don’t feel misled, these children carry the virus and even show the same violent and cannibalistic behaviours as the rotting infected (‘Hungries’) that now rule the city, except the children aren’t actually deceased like these. They appear alive and cognitive after inheriting the gene from their parents in utero, except with a fairly disturbing and gruesome maturation process. The virus itself is a ‘Day of the Triffids’ homage, in that it’s a fungus which destroys the host but somehow reanimates them as a decomposing cannibal before growing into a tree and spreading its spores as an airborne bacteria.
The writer Mike Carey, who has written several comic books, films and TV series, uses Schrödinger’s theory of a dead/live cat in a box as the metaphor for whether the children are still human. Glenn Close, playing the sinister child-killing scientist, explains to the girl they’re planning to dissect, ‘the logical answer is that the cat is both dead and alive’.
However,I believe this theory often gets misinterpreted as a metaphor for morality and existence,  when it’s really about physics although it often gets discussed in philosophy. Carey also has the girl’s guardian, a young teacher employed to observe the children’s behaviour, recite them a story about Pandora’s Box. She teaches them the moral lessons which the scientists and soldiers have apparently forgotten.