Is rabies vaccine only effective if antibodies develop before virus reaches nerves? or is it effective if antibodies develop before reaching cns?

No, Vaccines are : given before infection occurs to protect people working with animals, labs, travelers to a remote area that may have potential exposure to animals that can be rabid, and also after exposure to a potentially rabid animal where saliva and skin laceration occurs. They are just given at different interval sequencing. Both types of prophylaxis is protective against potential exposure to the virus.