The U.S. will probably run out of adults who are enthusiastic about getting vaccinated within the next two to four weeks, according to a KFF analysis published on Tuesday. Vaccine hesitancy is rapidly approaching as our main impediment to herd immunity, according to a recent feature from Axios.

"It appears we are quite close to the tipping point where demand for rather than supply of vaccines is our primary challenge," the authors of the analysis wrote. "Federal, state, and local officials, and the private sector, will face the challenge of having to figure out how to increase willingness to get vaccinated among those still on the fence, and ideally among the one-fifth of adults who have consistently said they would not get vaccinated or would do so only if required."

With supply having increased significantly and eligibility fully open to adults in all states as of April 19, the main question has become, when will supply outstrip demand? While timing may differ by state, we estimate that across the U.S. as a whole we will likely reach a tipping point on vaccine enthusiasm in the next 2 to 4 weeks. Once this happens, efforts to encourage vaccination will become much harder, presenting a challenge to reaching the levels of herd immunity that are expected to be needed.

According to the Kaiser polls, and others, have shown that the share of adults who have either received one vaccine dose or want to get vaccinated as soon as they can has continually increased. As of March 21, it was 61 percent (up from 55 percent the month before). This increase reflects a shift from those saying they want to “wait and see” into the vaccine enthusiasm group. In fact, the share saying they want to “wait and see” has consistently fallen, as more people become enthusiastic about getting vaccinated.