Comparison of immunogenicity of two hepatitis A vaccines Ð VAQTA 1 and HAVRIX 1 Ð in young adults Yaa Ashur, Ruth Adler, Mina Rowe, Daniel Shouval * Liver Unit, Division of Medicine, Hadassah University Hospital, POB 12000, 91120 Jerusalem, Israel Received 1 July 1998; received in revised form 25 November 1998; accepted 25 November 1998 Abstract Two new hepatitis A vaccines have been developed, and their immunogenicity tested using dierent immunoassays. The present study was designed to compare the immunogenicity of these two hepatitis A virus (HAV) vaccines Ð VAQTA 1 and HAVRIX 1 Ð as determined by seroconversion rates and anti-HAV titers, and using the same immunoassay. Healthy volunteers (15±30 y), seronegative for anti-HAV, were randomized in an open single center study to four groups of 20±21 vaccinees each, to receive either a 25 U or a 50 U dose of VAQTA, or HAVRIX at 720 EU or 1440 EU/dose, administered at 0, 1 and 6 m or at 0 and 6 m, respectively. Four weeks after primary immunization, seroconversion rates were 100% for VAQTA and 95% for HAVRIX, following injection of 50 U or 1440 EU, respectively ( p = NS) and anti-HAV GMTs were 40 and 37 mIU/ml for VAQTA and HAVRIX, respectively. At 6 months, prior to the booster dose, seroconversion rates were 100% for both vaccines, with anti-HAV GMTs of 111 and 70 mIU/ml for VAQTA and HAVRIX, respectively (P < 0.05). At month 7, four weeks after the only booster injection, using the two dose regimen, anti-HAV titers were 2212 and 1511 mIU/ml for VAQTA and HAVRIX, respectively (P < NS). Using three doses of 25 U/dose of VAQTA or 720 EU/dose of HAVRIX at 0, 1 and 6 m did not produce any clinically evaluable advantage over the two dose regimen for either vaccine. No signi®cant adverse events were observed using either vaccine. In summary, both vaccines have similar immunogenicity demonstrated using identical immunoassays for evaluation. These results also con®rm the outstanding immunogenicity of a single dose of either of the HAV vaccines and support their use in pre- and possibly postexposure prophylaxis against hepatitis A virus infection. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Hepatitis A; Vaccines; Single dose immunization 1. Introduction Hepatitis A virus (HAV) infection is a disease with a world wide distribution. Although rarely fatal, it may cause signi®cant morbidity, especially in adults and older people. Recent changes in world wide epidemiol- ogy of HAV infection are associated with the emer- gence of a progressively larger adult population susceptible to infection [1, 2]. In the past 10 years, two highly ecacious vaccines have been developed against HAV infection [2±5]. Both vaccines, VAQTA (Merck and Co., Inc., West Point, PA, USA) and HAVRIX (SmithKline Beecham Biologicals, Rixensart, Belgium), were developed from attenuated CR 326F, F 0 and HM-175 HAV strains, respectively, grown on MRC-5 human diploid ®broblasts and inactivated with formaldehyde [3±8]. Currently, clinically tested and licensed doses of the vaccines for pediatric and adult use include 25 and 50 units of VAQTA (in 0.5±1.0 ml), corresponding to 0 25 and 50 ng of protein. HAVRIX is manufactured with 360 (for pediatric use), 720 (for pediatric and adult use) and 1440 (for adult use) EU (ELISA units) suspended in 0.5±1.0 ml. It is dicult to compare the exact HAV protein content of Vaccine 17 (1999) 2290±2296 0264-410X/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0264-410X(98)00480-0 * Corresponding author. Tel.: +972-2-677-7337; fax: +972-2-642- 0338; e-mail: shouval@hadassah.org.il