Herd immunity occurs when a suffcient percentage of a population has become immune， reducing the likelihood of infection for individuals who lack immunity.

= Herd immunity occurs when a suffcient percentage of a population has become immune, which reduces the likelihood of infection for individuals who lack immunity.

reducing...前应该有逗号。reducing...现在分词短语的逻辑主语为前面划线的部分，为when从句的结果状语。这个分词短语可以改写为which引导的关系分句，which的先行词为划线部分。