The first step uses a bit of influenza DNA to get the patient's immune system to begin fighting off the flu, and then the protection is given a "boost" some time later by the second step, which can be either another bit of DNA or a conventional flu vaccine.

VOA: standard.2010.07.21

You have to inject it multiple times because the first blip that you get in immune response is not very high, you have to boost and often you have to boost again in order to get a high enough response to be protected.

你必须得进行多次注射,因为第一次注射,无法使免疫系统产生强烈反应,你得要增强免疫,通常还得再增强一次,以使反应足够强烈,从而起到保护作用

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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