Thanks to vaccination, public health has improved significantly in developing countries.

vaccination 是名词，由动词 vaccinate（接种疫苗）加上名词后缀 -ion 构成。vaccinate 本身来自名词 vaccine（疫苗），表示"接种疫苗"的行为或过程。vaccination 指"疫苗接种"这一医疗行为。