Around 120 million American adults still haven’t received any of the shots. When unvaccinated poll respondents were asked whether they planned to do so, 41 percent said yes. But 56 percent said no. That “no” group of people will receive increasing attention in the coming weeks, as public health officials puzzle over how to keep the country moving toward the goal of widespread immunity, where so few people are able to spread the virus that its transmission is dramatically stemmed.