Children everywhere still need to be immunized, and children whose immune systems are weak,sometimes get a type of polio that's shed from the vaccine into sewage.
世界各地的儿童仍然需要免疫接种，而免疫系统薄弱的儿童有时会患上一种脊髓灰质炎，这种病会从疫苗中扩散于污水中。

that's shed from the vaccine into sewage. =

that has shed from the vaccine into sewage. （a type of polio 的定语从句）

是现在完成时，不是被动语态。

结构：shed（不及物动词） from A into B（状语）.   意思： 从A 流入B；从A 扩散到B