- Background: Several studies have demonstrated the efficacy of hepatitis A virus (HAV) active vaccine in the prevention of secondary HAV infection when administered shortly after exposure. Methods: We describe six new recruits with unknown HAV infection, of whom three received late postexposure prophylaxis with the active HAV vaccine and three were not vaccinated. Results: Results indicated that the vaccinated patients had a longer period from exposure to onset of symptoms (p < 0.05), shorter hospitalization, and lower liver enzyme levels. Conclusions: It appears that late administration of the active HAV vaccine has a disease-modifying effect. These findings, combined with earlier reports, may have important implications for immunization policies.