The U.S. Is Entering a New COVID-19 Vaccination Crisis

In the past three weeks, every adult in the U.S. has become eligible for COVID-19 vaccines, which are now widely available in most parts of the country. Yet there has been an alarming decline in the number of Americans showing up to get vaccinated, even though less than half of the population has received even a single dose. While data on the progress of the vaccine rollout are difficult to parse given the many moving pieces, this is almost certainly a sign that a large number of adults remain vaccine hesitant.

In the first several months of the rollout, as states debugged the complex logistics of distributing their allotted vaccines, the population of people eager for a shot vastly outnumbered the awaiting syringes. Now, supply clearly outweighs demand. After cresting at over 2 million on April 13, the number of people receiving their first dose of a vaccine each day—the best metric to show real-time vaccine hesitancy—has stood below 1 million for more than a week:

Before declaring a crisis in vaccine hesitancy, let’s consider an alternate explanation: Could there be a bottleneck in availability as a vastly larger population of people have become eligible? Unlikely. The total number of doses allocated to the states each week by the U.S. Department of Health and Human Services has remained steady at about 18 million since early April, and states have recently been using around 75-80% of the doses they have ordered. In fact, some states are now ordering fewer doses than they are being offered, as the New York Times recently reported, suggesting a drop in demand.

There’s another twist to the vaccination slowdown: the data strongly suggest that an increasing number of people are only showing up for their first dose of the two-shot Pfizer-BioNTech or Moderna vaccines, and forgoing their second shot. Through April 9, 8% of first dose recipients who were eligible for their second shot had not yet received it, the CDC said last month. That amounted to 5 million people at the time.

While the CDC doesn’t regularly report this figure, there’s another way we can estimate this halfway hesitancy. Given the typical three- to four-week delay between people’s first and second Pfizer or Moderna doses, the number of people who have received their second dose lags behind that of those who have received both. If 100% of first-dose recipients were showing up for their second dose on time, we’d expect there to be a lag time of around 25 or 26 days—an average of the typical 21-day and 28-day wait times for Pfizer and Moderna, respectively, weighted slightly toward 21 days since Pfizer accounts for more of the doses administered so far. This delay gets longer if more people don’t show up for their second dose, since those who only receive one shot do not contribute to the second-dose rate.

Considering this, we can estimate the “incompletion rate” by measuring how long it’s taking the completion rate to catch up to a given day’s first-dose rate. The longer that takes, the more people we can estimate are not following through. At its lowest point, on Feb. 11, the lag was 27 days. By April 9 it was 30 days. As of writing, it is 34 days. (This calculation does not include the one-dose Johnson & Johnson drug, which accounts for 9 million, or 3.4%, of the shots administered so far.)

Hesitancy, it seems, comes in a least two flavors: Those who are doubtful of the entire process, and those who figure one dose is plenty enough. While a single dose offers some protection, public health officials urge people to get both doses of the two-shot vaccines to ensure maximum efficacy and longevity.

Less clear is what’s driving the overall decline in first-dose recipients. Polls indicate that those who characterize their attitude towards vaccination as “wait-and-see” has declined from 39% in December to 15% in April. Meanwhile, the percentage of people who are entirely unwilling to get a shot or will only do so if required has barely changed, from 24% to 19%, in the same time span.

In theory, those data points are a sign that more people are feeling ready to get their shot—yet daily vaccinations are plateauing. The best explanation I have is what a good friend of mine, who is also the person editing this article, calls vaccine “meh-sitance.” It’s a hassle to get vaccinated, even if there’s a pharmacy down the street offering the shot. We’re all guilty of procrastinating on any errand that doesn’t feel urgent, particularly if it involves a process of suboptimal efficiency. COVID-19 case rates and mortality rates in the U.S. are way lower than they were during the holidays, and a lot of other people are already vaccinated, so what’s the rush?

We know the answer, of course: 46% of the population is not nearly high enough to push the pandemic to endemic levels, much less eliminate it altogether. Moreover, vaccination remains the best way for a given person to protect themselves from COVID-19. The best possible explanation for the decline is that there is a corresponding drop in motivation, not a core willingness to ever get the vaccine. No one likes waiting in line. Even if the line ought to be longer.

Note on the data: The first graph, tracking those who are receiving a first dose, isn’t quite the same statistic as those receiving an “initial dose,” which we update daily on TIME’s vaccination dashboard, since Johnson & Johnson recipients normally count toward the same column as those receiving both doses of either Pfizer-BioNTech or Moderna. This is a small difference, as nearly 152 million people have received at least the first dose of Pfizer or Moderna, which were approved three months earlier.