http://www.newsweek.com/when-will-we-get-herd-immunity-covid-vaccines-1579451

The number of people to have received at least one vaccine dose against COVID in the U.S. is approaching 150 million—but it is still uncertain when the U.S. will reach herd immunity.

Herd immunity refers to the fact that having people immune to a virus in a given population will slow down the virus' spread through that population. The more people in that population who are immune, the slower the virus will spread.