Jennifer Simon, an elementary school speech language pathologist, poses Feb. 25, 2021, in Nashville, Tenn., to show the adhesive bandage marking where she received her COVID-19 vaccination. (AP Photo/Mark Humphrey)

The COVID-19 vaccine campaign in the Chattanooga region has begun to shift away from supply-and-demand challenges that marked early efforts and will face a new challenge in the coming months — how to get doses in the arms of those who haven't yet received them.

Vaccine appointments at the Hamilton County Health Department are no longer filling up moments after they're posted. After announcing new first-dose slots Wednesday morning, health department officials issued a second alert around 5:30 p.m. that hundreds of appointments were still available Thursday at the Enterprise South location for those who signed up before midnight.

Even more appointments sit unfilled in surrounding rural counties, according to state health officials.

"Uptake of the vaccines in Tennessee remains low across all racial groups, especially in rural areas. Many rural Tennessee county health departments are seeing only a fraction of their daily COVID-19 vaccination appointments filled," a news release from the Tennessee Department of Health issued Tuesday states.

About 50% of vaccine appointments in East Tennessee are filling up, whereas about 80% of appointments in Middle Tennessee are booked, according to the news release.

Most appointments in West Tennessee counties are going unfilled, with about 20% of daily appointments currently booked.

(READ MORE: Tips for navigating Tennessee's new vaccine eligibility)

The Biden administration's goal is for all adults to be able to access vaccines by May, and supplies have ramped up significantly in recent weeks.

"TDH [the Tennessee Department of Health] is developing a strategy to accelerate through phases of the vaccination plan while balancing vaccine supply and demand," Tuesday's news release states.

Some states, including Mississippi and Alaska, already have opened up vaccine eligibility to anyone age 16 and older. However, some public health experts caution that offering vaccines too soon could leave vulnerable populations behind.

In Tennessee, more than 1.3 million Tennesseans have received at least one dose of COVID-19 vaccine and more than 2 million doses of vaccine have been administered so far. However, the state falls near the bottom of all states in terms of the percentage of the population fully vaccinated.

Roughly 10.4% of Tennesseans are fully vaccinated — or 701,325 residents — according to the Johns Hopkins COVID-19 vaccine tracker as of Wednesday.

Alabama has vaccinated 10.3% of its population, and Georgia — which is trailing all other states in percent of population vaccinated — has vaccinated 10.1% of its population, according to Johns Hopkins.

Alaska has vaccinated 19% of its population, more than any other state. The U.S. average is 11.7%.

Scientists estimate that a vaccination rate of about 70% is needed in order to achieve herd immunity against COVID-19.

Though polling shows more Americans than ever want vaccines, experts anticipate reaching herd immunity will be an uphill battle given vaccine hesitancy, barriers to access and historical distrust of vaccines among certain populations.

Within weeks, the White House plans to launch a $1.5 billion public relations campaign with the goal of convincing more people to get vaccinated, according to the health and medical news site STAT.

Pro-vaccine ads tailored to young people, people of color and conservatives — three groups for which vaccination rates are generally lower — will span television, radio and digital media, according to STAT. But some experts question whether the Biden administration can sway vaccine-skeptical conservatives.

On Tuesday, Tennessee Health Commissioner Dr. Lisa Piercey spoke to a crowd at a mass vaccination site in Nashville about the waning demand for vaccines. Piercey said people who can relate to those hard-to-reach communications and vice versa make the best messengers to combat vaccine hesitancy.

"I can relate to them, because they're like me," she said. "If they say it's OK, I'm more likely to believe that."

Contact Elizabeth Fite at or follow her on Twitter @ecfite.