联邦官员周四报告说，一份剂量的新型疫苗可以充分保护接种人免于感染猪流感，以前预计的疫苗剂量则为两份。
On Thursday, federal officials reported that one dose of the new vaccine, rather than the two they had expected, appeared to fully protect recipients against swine flu.
小剂量的硒元素对人的健康是非常有用的，它能使人形成一个强大的免疫系统，防止感染和疾病的发生。
Small amounts of selenium are essential for health and help build a strong immune system to fight off infections and diseases.
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