ACIP recommends that healthcare personnel with written documentation of having received a properly spaced series of HepB in the past (such as in infancy or adolescence) but who now test negative for anti-HBs should receive a single “booster” or “challenge” dose of HepB and be retested 1–2 months later. Those who test positive following the “booster” dose are immune and require no further vaccination or testing. Those who test negative should complete a second 2- or 3-dose series of HepB on the usual schedule and be tested again 1–2 months after the last dose. The “booster” dose counts as the first dose in this series. Heplisav-B or PreHevbrio may be used to revaccinate new healthcare personnel (including the challenge dose) initially vaccinated with a vaccine from a different manufacturer in the distant past who have anti-HBs less than 10 mIU/mL upon hire or matriculation. For more information see www.cdc.gov/mmwr/volumes/67/rr/pdfs/rr6701-H.PDF, pages 21–22.