VOA慢速英语:我们怎么知道什么时候达到群体免疫？

variant - n. something that is different in some way from others of the same kind

immunity - n. the power to keep yourself from being affected by a disease

antibody - n. a substance produced by the body to fight disease

update - v. to change (something) by including the most recent information

flu - n. a common disease that is caused by a virus and that causes fever, weakness, body aches, and breathing problems