This is basically why I would save her.
It’s not a simple question of saving Ellie vs. everyone else (in which case I would most likely concede that she is worth the sacrifice).
It's not your call to sacrifice a human even if it's to save others. Ellie is healthy and sacrificing her as you put it is murdering her. I guess we all have our opinions but let me tell you that yours is horrible... If happening in reality of course. But the way you and others seriously answer this question makes me think you mean this, should a similar situation happen in real life.
If you need to spill blood of the innocent to allow others to live, that voids any kind of decency those people could have. I would HATE to know that someone, especially a child had to be sacrificed so I could live. It is so fundamentally wrong that I staunchly reject such a notion.
I'm in the zone, don't bother me!