The immunity achieved is generally brief and must be boosted. 所獲得的免疫性一般是短暫的,須激發(fā)注射。
Hepatitis A is endemic in most developing countries, and most children are exposed and develop immunity at an early age. 型肝炎是大多數發(fā)展中國家的一種地方病,而大多數孩子因為在很小的時(shí)候曾患病而具有免疫性。