if mother is hbsag positive she infects the baby and hbv becomes cronic more than 95% cases, in order to prevent infection very close to 100% vaccine only is not enough if hbeag is positive and hbvdna detactable so now infected mothers take tenofovir antiviral before delivery to get hbeag negative and hbvdna undetactable so that chances vaccine might fail are very close to zero
if mother is already hbeag negative and hbvdna undetactable antiviral is probably not necessary as long as this state persists until delivery
Have your Girlfriend consult her OBGYN now on everything I've read recently even if a baby is born to a HBsAG positive Mother meaning an infectious mother the Baby can be given a vaccine that will destroy the infection @ Biirth. Check this with the OBGYN. Best Wishes.