Hepatitis A is a persistent but not lifelong immunity, and only a small percentage of the population is immune for life. In the field of epidemiology, people susceptible to hepatitis A (HAV) are mainly those who are anti-HAV negative in their blood, so the absence of antibodies in the body is when a person is susceptible to hepatitis A. In the adult population, the presence of anti-HAV IgG (i.e., antibodies that neutralize the hepatitis A virus) is a major factor. The detection rate of anti-HAV IgG (i.e., antibodies that neutralize the hepatitis A virus) is as high as 80% in the adult population. Due to individual differences, the appearance and duration of anti-HAV IgG varies widely. In general, anti-HAV IgG peaks 2-3 months after HAV infection and persists for many years in the majority of humans, although it may be present for life. The body is not susceptible to hepatitis A when anti-HAV IgG is present in the body at all times, but there is still a risk of infection when anti-HAV IgG is absent or diminished in the body.