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.

American biotechnology company Regeneron Pharmaceuticals announced Wednesday that its cocktail of two monoclonal antibodies held up in laboratory experiments against new variants of the novel coronavirus first identified in the United Kingdom and South Africa.

Monoclonal antibodies are synthetic versions of our natural antibody defense to infection. They are being studied as a way to both treat and prevent COVID-19 infection, with promising results. But unlike vaccines, which are thought to offer broader protection, some scientists have been worried that this type of therapy would be less effective against newly emerging variants of the virus.

Wednesday's announcement is good news for Regeneron's monoclonal antibody treatment, REGEN-COV, though the data is still preliminary and currently under peer review.

Regeneron scientists as well as researchers at Columbia University in New York City have each independently confirmed that the casirivimab and imdevimab antibody cocktail successfully neutralized both the U.K. and South Africa variants when tested against them, according to a company press release.

REGEN-COV has not yet been tested against another variant that was first identified in Brazil. However, Regeneron said the two-antibody cocktail "is expected to remain similarly potent" based on some resemblance which the Brazil variant bears to the South Africa strain. The company said it is pursuing further confirmatory research.

It's the latest piece of promising news about the efficacy of monoclonal antibodies as treatment for COVID-19. On Tuesday, American pharmaceutical firm Eli Lilly and Company announced that a combination of two monoclonal antibodies, bamlanivimab and etesevimab, was found to be effective in COVID-19 patients at high risk of severe infection, reducing the risk of hospitalization and death by 70%, according to the results of a final-stage trial.

That same day, Regeneron announced its antibody cocktail had shown positive initial results in prophylactic use -- that is, helping ward off COVID-19 in those who may have been exposed to the virus. Regeneron’s chief scientific officer, Dr. George Yancopoulos, said he hopes the drug "may be able to help break this chain" of active infection and transmission.

Last Thursday, Eli Lilly released data showing bamlanivimab may help prevent disease and stop outbreaks among residents and staff of long-term care facilities.

ABC News' Sasha Pezenik contributed to this report.