Assistant Secretary of Defense for Public Affairs John Kirby tweeted that the Pentagon is "pausing" the plan to move forward with vaccinations for detainees at Guantanamo Bay.

"We’re pausing the plan to move forward, as we review force protection protocols," Kirby said. "We remain committed to our obligations to keep our troops safe."

No detainees have been vaccinated yet, he said.

ABC News’ Molly Nagle contributed to this report.

Hard-hit California has surpassed 40,000 deaths from COVID-19, according to the state’s Department of Public Health.

At least 40,216 people in the Golden State have lost their lives, which equals about 1 in every 1,000 Californians.

ABC News’ Matt Fuhrman contributed to this report.

Nashville's Music City Center opened Saturday as a mass vaccination site.

Those 75 and older who already have an appointment booked can get vaccinated at the concert venue, according to ABC Nashville affiliate WKRN.

"We think that we’ll be able to do up to 1,000 first doses here and up to 1,000 second doses here a day, once we receive enough vaccine for that,” Metro Nashville Health Department Interim Medical Director Gill Wright told WKRN.

One case of the South African B.1.351 variant has been confirmed in the Baltimore metro region, Maryland Gov. Larry Hogan said.

"The individual has not traveled internationally, making community transmission likely," Hogan’s office said. "Comprehensive contact tracing efforts are underway to ensure that potential contacts are quickly identified, quarantined, and tested.

"The B.1.351 variant has not been shown to cause more severe illness or increased risk of death when compared to other variants. The variant is believed to be more transmissible than other strains," Hogan’s office said. "Additional research is still required to determine the effectiveness of available vaccines against the B.1.351 variant. However, initial evidence suggests that vaccines are still likely to be protective against the variant."

The United States’ first cases of the South African variant were confirmed this week in two people in South Carolina.

Dr. Anthony Fauci, chief medical adviser to U.S. President Joe Biden and the nation's leading infectious disease expert, said Americans can expect to see more COVID-19 vaccine doses available throughout the country in the coming weeks.

"As we get into February, March and April, we're going to see an escalation of availability of doses that we may have not had a week or two or three ago," Fauci, who is also the director of the National Institute of Allergy and Infectious Diseases, told ABC News chief anchor George Stephanopoulos Thursday on "Good Morning America."

Although COVID-19 data is starting to show some promising trends, Fauci said the U.S. outbreak is "still a very serious situation."

"If you look throughout the country, the dynamics of the outbreak are a little bit unstable," he noted, "even though for the most part we're seeing general trends, seven-day trends of cases going down and ultimately, hopefully, hospitalizations and deaths."

When asked about reopening schools safely, Fauci said COVID-19 transmission rates in schools actually appear to be less than in communities when the schools have the resources and protection they need.

"So if you have a situation where you have dynamics of a viral spread in a community, it's less likely that the children who are in the school -- if we do things right, it's less likely that they're going to get infected," he said. "Obviously we want to get the teachers vaccinated, we want to make sure they have the resources to do it correctly. But when you look at the history of how this virus is moving in schools, it seems to be less spreading there than it is in the community."

Fauci also expressed concern over the new, more contagious variant of the novel coronavirus that was first identified in South Africa and has since spread to dozens of other nations, including the United States.

"The one in South Africa, George, troubles me," he said, explaining that lab experiments show the neutralizing antibodies induced by existing COVID-19 vaccines are "diminished by multifold" when tested against the South Africa variant, called B1351.

"It's still within the range of what you would predict to be protective," he added, "but I take no great comfort in that."

However, scientists are already working on vaccines that will specifically target the South Africa strain, according to Fauci.

"May not be necessary," he said, "but if it is we'll already be on the road to be able to give people a boost that directs against the South African isolate."