At the same time, the share of adults who say they are unsure (7 percent), are waiting to see “what happens to others before deciding” (10 percent) or will “never” get vaccinated (20 percent) has not budged. The implication is stark: Unless many of the unvaccinated Americans who have been saying for months that they’re waiting or unsure have a sudden change of heart, fewer than 65 percent of U.S. adults are likely to be inoculated against COVID-19 this spring — far short of the level experts say is required for the kind of lasting population-wide protection known as herd immunity. Reaching that threshold — the point when an estimated 80 percent of all Americans regardless of age have been vaccinated and the coronavirus runs out of potential hosts — would then require an additional 96 million U.S. residents to be vaccinated, a process that could presumably begin when regulators approve the COVID vaccines for use in children.