In order to assess the preventive and immunological effects of the newly developed hepatitis A vaccine, 7-year-old susceptible children in a village were immunised with the hepatitis A vaccine. After 3 years of follow-up observation, the results were as follows.<image 1> The main outcome of the study was
['Incidence of hepatitis A', 'Prevalence of hepatitis A', 'Secondary attack rate of hepatitis A', 'Survival rate of hepatitis A']