Maureen Condic’s essay at the Witherspoon Institute is a combination of philosophy, bioethics, and science fiction. It’s worth a read. Here is her conclusion:
Stated in more philosophical terms, “rational animal” is the essential definition of a human being, and “developing human” is the essential definition of a human being at embryonic stages. Therefore, if an entity is developing and of human origin it is a human being by definition, regardless of any defect it may exhibit in brain formation. Thus, if a human embryo infected by the zombie pathogen subsequently died and was resurrected, the nature of the resulting entity would depend entirely on what happened next. If the entity proceeded through embryonic development (an unlikely scenario), it would be, in virtue of this fact alone, a human being. And just as is the case for anencephaly, any impact the pathogen had on subsequent formation of the brain would reflect a defect in a part, not an alteration in the nature of the entity. Given that human beings at embryonic stages of life are defined by their capacity to undergo development, it is not possible, by definition, to engineer a developing human zombie; i.e., an embryo of human origin that utterly lacks the capacity to produce a brain capable of rational thought.