After months of a steady climb in vaccinations, the U.S. is experiencing its first true slowdown in the rate of daily shots, an indication that the nation is entering a new phase of its inoculation campaign.

More than half of American adults have now received at least one dose, a significant achievement, but getting the second half vaccinated brings a different set of challenges than the first. Vaccinations to date are likely made up largely of groups who most wanted the vaccine and have had greatest access to it, and continued progress is no longer about meeting demand for shots.

“We’ve gotten vaccinations to those most at risk and those most eager to get vaccinated as quickly as possible,” White House Covid czar Jeff Zients told reporters last week, “and we will continue those efforts, but we know reaching other populations will take time and focus.”

The country is averaging 2.6 million reported vaccinations per day over the past week, Centers for Disease Control and Prevention data shows, down from a peak of 3.4 million reported shots per day on April 13. That number is trending downward even with eligibility now open to all adults in every state.

The downturn comes on the heels of lots of positive vaccination news, said Dr. Jennifer Kates, a senior vice president with the nonprofit health policy group the Kaiser Family Foundation. The federal government has secured a large vaccine supply, states have ironed out kinks in their registration systems, and eligibility has opened up to all adults.

But now the country is approaching the point where meeting existing vaccine demand is no longer the main challenge, according to Kates.

“We feel like we’re getting to the point where the people that are left are very hard to reach and need assistance and more education, or those that are resistant and don’t want it,” she said, adding that “the pent up demand is being met.”

The question ahead is what the response to the slowdown will look like. “How does the federal government, state government, and the private sector communicate to the public the importance of vaccination?” Kates asked.

States see demand slowdown
In tandem with the nationwide slowdown, many states are seeing their vaccination paces slide. Eleven states have reported a decrease in shots given for three weeks in a row or more, according to a CNBC analysis of CDC data through Sunday.

In West Virginia, which got off to a hot start with their vaccination campaign, the state has passed the tipping point of vaccine supply outpacing demand. Weekly doses administered have dropped for four straight weeks.

“If you remember, we put a lot of doses in a lot of arms really fast,” said Maj. Gen. James Hoyer, director of West Virginia’s Joint Interagency Task Force for Vaccines, noting that his state was among the first to finish vaccinations among nursing home populations. “It was a lot of people who wanted them and were pushing hard to get out and get a vaccine.”