Concerns over waning immunity and SARS-CoV-2 variants have convinced some countries to deploy extra vaccine doses — but it’s not clear to scientists whether most people need them.

中文翻译：

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COVID疫苗助推器：最重要的问题

对免疫力下降和 SARS-CoV-2 变体的担忧已说服一些国家部署额外的疫苗剂量——但科学家们不清楚大多数人是否需要它们。