The minimum interval between the two doses of Heplisav-B is 4 weeks. For the 3-dose series vaccines, Engerix-B, PreHevbrio, and Recombivax HB, the minimum interval between the first and second doses is 4 weeks. The final dose of vaccine must be administered at least 8 weeks after the second dose and should follow the first dose by at least 16 weeks. Vaccine doses administered 4 or fewer days before the minimum interval or age are considered valid. Doses received 5 or more days before the minimum interval or age should be repeated using the correct schedule. Because of the unique accelerated schedule for Twinrix, the 4-day “grace period” does not apply to the first three doses of this vaccine when administered on a 0-, 7-, 21–30-day, and 12-month schedule.