If they are returning strictly for the vaccines, no other reason, then there wouldn't be another visit code. The visit was done on the previous visit. However, if the physician saw the patient for any other reason in addition to the vaccines, then you should bill that accordingly. The insurance company won't pay twice because the patient, for whatever reason, didn't get the vaccines at the original visit. I have had this happen with my own children. Ran out of vaccine, we had to return a week later. Only saw the nurse, received the vaccine, and only the vaccine was billed.
Michele