Some of the newly vaccinated said they made the decision abruptly, even casually, after months of inaction. One woman in Portland, Ore., was waiting for an incentive before she got her shot, and when she heard that a pop-up clinic at a farmers’ market was distributing $150 gift cards, she decided it was time. A 60-year-old man in Los Angeles spontaneously stopped in for a vaccine because he noticed that for once, there was no line at a clinic. A construction worker said his job schedule had made it difficult to get the shot.

Many people said they had arrived for a vaccine after intense pressure from family or friends…

Others were moved by practical considerations: plans to attend a college that is requiring students to be vaccinated, a desire to spend time socializing with high school classmates, or a job where unvaccinated employees were told to wear masks. Their answers suggest that the mandates or greater restrictions on the unvaccinated that are increasingly a matter of debate by employers and government officials could make a significant difference.