The U.S. appears to be reaching its ceiling on COVID-19 vaccinations, at least among adults.

Why it matters: The more transmissible and dangerous variant Delta variant is spreading fast, and experts fear another wave of infections among the unvaccinated.

"We’ve hit the wall in the number of vaccinations in recent weeks," Eric Topol, director and founder of the Scripps Research Translational Institute told Axios.

• "Just trying to deduce from other countries where we're headed, if we don't get a big jump up in our vaccination rate, we're going to be vulnerable for a lot more cases," Topol said, adding that comes with increased hospitalization, long COVID and death.

Driving the news: President Biden yesterday all but begged the unvaccinated to get a shot, reminding them that it's free and telling them to think about the family members they might put at risk by not getting vaccinated.

• The Americans who were most receptive to that message, however, are already vaccinated.

By the numbers: Roughly 67% of American adults have had at least one shot, and 58% are fully vaccinated, according to the CDC.

• There's no magic number that determines herd immunity. Experts put out estimates between 60% to 70% early in the pandemic, but they've since revised those estimates, in some cases to at least 85%, with the increase in variants.
• The U.S. hit its peak in April, when CDC data shows more than 4.4 million doses were distributed in a single day. The nationwide average has now fallen to about 500,000 shots per day.

“If we continue to let this pandemic run wild... there is a probability that there will eventually be a variant against which the vaccines will be less effective," Saad Omer, director of the Yale Institute for Global Health said in a blog earlier this year.

What we're watching: The U.S. will continue to wind down its mass vaccination sites, shifting to more targeted efforts such as distributing vaccines through doctors' offices, pharmacies, mobile clinics, and employers., as well as a door-to-door effort to persuade vulnerable Americans, Biden said yesterday.

• One of the most at-risk groups who need targeting includes young adults between the ages of 18 and 30 years old who "feel bulletproof," Cornell virologist John Moore told Axios.
• They need to hear the message that "this bullet could hit you," Moore said.

Beyond that, the U.S. is simply fighting an uphill battle as vaccine attitudes continue to track with political sentiment. Moore pointed to a dearth of celebrities encouraging the vaccine, likely because they worry about the fallout.

• Topol pointed to two additional groups: those who'd get a vaccine if mandated by their employer and those who say they've held back on getting a vaccine while it's still under emergency use authorization.
• In both cases, full approval of the COVID-19 vaccine by the FDA would move things along. Schools, businesses and even the military have been reluctant to mandate shots under an EUA, even though the federal government has said it is allowed.
• The gap between an EUA and full authorization is a "distinction without a difference," Moore said, but some people who don't understand the process may feel more comfortable with full authorization.

The FDA did not respond to a request for comment about when the agency might fully approve the COVID-19 vaccine created by Pfizer, which was the first to gain an EUA.