Joyce Ann Kraner is eager for the pandemic to end and for life to get back to normal. Kraner, 49, wants to be able to hug her mother, who lives in a nursing home.

But she says she has no plans to get the vaccine, even though it’s widely available in her community of Murfreesboro, Tenn. “I feel like I’m healthy,” she says.

Despite the fact that millions of people have been vaccinated safely, Kraner worries about complications. She believes some people are having “life-threatening reactions” to the vaccine that the media aren’t reporting. (Many such reports shared on social media are false or misleading.) And she’s worried because it’s so new: “We don’t know the long-term effects. We don’t know what it’s going to do.”

A recent NPR/Marist poll found that one in four Americans said they would refuse a coronavirus vaccine outright if offered. Another 5% are “undecided” about whether they would get the shot. Although the numbers were highest for Republican men and residents of rural areas, there were still a significant number of people across all ages and demographic groups who claim they will say “no.”

Now some researchers are increasingly worried that this reticence will be enough to prevent the nation from reaching what’s known as herd immunity, the point at which the coronavirus can no longer spread easily through the population and transmission peters out. Reaching high levels of vaccination would mean new outbreaks of the coronavirus would die down quickly, as opposed to growing and spreading.

“Vaccine hesitancy is a big problem for all of us,” says Ali Mokdad, who tracks coronavirus trends at the Institute for Health Metrics and Evaluation at the University of Washington.

Up until now, the nationwide vaccine campaign has seen demand outstrip supply, but Mokdad says he believes that will soon change. By May, he believes, “We will have more vaccines than people willing to take the vaccine.”

It’s hard to know exactly how many people will choose not to get vaccinated. NPR/Marist’s polling has seen the number of people saying they would refuse a vaccine drop since the question was first asked in August, and it continues to fall.