A randomized study of a flexible booster dosing regimen of VAQTA (R) in adults: safety, tolerability, and immunogenicity

Background: VAQTA (R) (hepatitis A vaccine inactivated, Merck & Co., Inc., West Point, PA) is licensed for use in healthy adults in a two-dose schedul e at 0 and 6 months. Objective: to determine whether the responses to a boo ster dose of VAQTA (R) administered to adults 12 or 18 months after the fir st dose were similar to the response when the booster dose was administered 6 months after the first dose. Methods: healthy adults were randomized to receive 50-U of VAQTA (R) at 6 (Group I), 12 (Group II), or IS months (Grou p III) following receipt of Dose I on Day 0. Blood samples were collected i mmediately prior to Doses 1 and 2 and then, 4 weeks following Dose 2. Serop ositivity rates (SPRs), geometric mean titers (GMTs) in milli-international units per milliliter (mIU/ml) and booster response rates (BRRs) were compa red among treatment groups. Safety data were collected on Vaccination Repor t Cards. Results: no serious adverse experiences were reported, and the vac cine was well-tolerated by subjects in the three treatment groups. One mont h following the booster dose, SPRs and GMTs for Groups I, II, and III, resp ectively, were, 100% (102/102) and 6726.4 mIU/ml; 97.9% (93/95) and 4863.8 mIU/ml; 100% (86/86) and 6068.3 mIU/ml. The BRRs were 88.2% (Group 1), 90.2 % (Group II) and 94.2% (Group III). Conclusion: responses to the booster do se were comparable regardless of the timing (i.e. 6, 12, or 18 months follo wing Dose 1). Flexibility in the timing of the booster dose of VAQTA (R) in adults would allow the vaccination schedule to be the same for adults, ado lescents, and children and may increase the likelihood that adults receive the booster dose. (C) 2001 Elsevier Science Ltd. All rights reserved.