Not enough time to make anti-bodies after the shot or not making anti-bodies at all
I haven’t got a full handle on the human immune system, but it seems like T-Cell immunity is the more important component.
http://www.differencebetween.net/science/difference-between-t-cells-and-antibodies/
FReepers please jump in to clarify or correct, but it looks like Antibodies are a last-ditch defence, only seen when a case is critical.
(Things were much simpler in the movie ‘Outbreak’!)