I think that what it means is that if a child is born to a mother who is HBP then they must recieve the vax within 72 hours for it to be an effective prevention against the baby contracting a full blown infection of the virus. (I think I posted this earlier, but it is about 90% of kids born to HBP mothers who contract the full virus) The actual makeup of the vax is the same, I believe, as it is when given at 2 months