short for vaccine: a substance put into a person's body to prevent them getting a disease

The three-year trial of the AIDS vax is intended to test whether it can protect people from infection from the H. I. V. virus. 为期三年的艾滋病疫苗临床试验是用来测试是否有可能预防被HIV病毒感染。

short for vaccination: the process or an act of giving someone a vaccine (= a substance put into a person's body to prevent them getting a disease)

Patient reported right-sided pain and feeling ill since vax. 患者报告说自从疫苗接种后，身体右侧疼痛及感觉恶心。