FDA approval of a Covid-19 vaccine will be only the start of a necessary series of events that should ultimately lead to individual-level protection, herd immunity, and a decline in transmission of infection. When most people develop immunity—either from previous infection or vaccination—it's less likely that an infected individual will encounter someone susceptible to the virus. Thus, widespread immunity would reduce the likelihood of transmission. And so, by slowing the spread of infection, widespread vaccination leads to herd immunity.
