They had to give some of them the real vaccine and some of them a placebo vaccine in order that they could really tell if the vaccine worked; you have to have it placebo controlled.

研究者们还须给部分孩子接种真正的疫苗,给另外一些孩子接种空白疫苗,以分辨疫苗是否真的有效,所以你必须使用安慰剂作对照

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