Some, like the smallpox vaccine on one exposure generates a very high response that lasts for many years so you don't need a boost.

有一些疫苗,比如接触一次天花疫苗后,人体就能产生高水平的免疫应答,并持续很多年,所以无需再次接种

耶鲁公开课 - 生物医学工程探索课程节选

Now, if I was a person that was designing a vaccine and I noticed that this was the response that it got, that antibodies were produced, they reached some intermediate level, they started to fall, I would say, "Well I haven't stimulated the immune system enough, let me re-boost, let me give another dose of antigen."

现在,如果我正在研究一种疫苗,我注意到机体会对这种疫苗产生应答,也就是会产生抗体,当抗体浓度达到中等水平后,就开始下降,就可以说,疫苗对免疫系统刺激还不足,我需要提高抗原浓度,让我再把剂量再加大一倍

耶鲁公开课 - 生物医学工程探索课程节选