- Abstract We evaluated in a prospective study the immune response of naïve subjects to a single dose of inactivated Hepatitis A vaccine. Ninety-seven percent of the vaccinees sero- converted 1 month after vaccination and 93% were still positive 2 years later. All of the vaccinees had a strong booster response 2 years after the single dose. Avaxim was more immunogenic than Vaqta for the primary dose (p= 0.01 for sero-positivity, p< 0.001 for antibody level) but no differences were found after boosting with Avaxim. Performance of intense physical activity during the first month after a single vaccine dose was associated with lower antibody levels (p= 0.004). This study indicates that a single dose of inactivated HAV vaccine elicits protective immune memory for at least 2 years.