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.

The United Kingdom now has the highest COVID-19 transmission and death rates of any country in the world, according to the World Health Organization's weekly epidemiological report released Wednesday.

With 383.1 new cases of COVID-19 per 100,000 residents, the U.K.'s transmission rate is higher than that of the United States, which decreased by 20% over the previous week to 380.6 new cases per 100,000 residents. But the UK appears to be moving in the right direction -- this week showed a 24% decrease in new cases confirmed from the previous week, the report said.

The U.K.'s COVID-19 death rate is also the world's highest, with 12.9 fatalities from the disease per 100,000 residents. The country -- which is made up of England, Scotland, Wales and Northern Ireland -- saw a 13% increase in new deaths over the previous week, according to the report.

In the U.S., the COVID-19 death rate went down by 7% to 6.5 fatalities per 100,000 residents, the report said.

As of Monday, 70 countries have detected cases of a new, more contagious variant of the novel coronavirus that was first identified in the U.K., while 31 nations have cases of another variant that first emerged in South Africa and eight countries have cases of a variant that was first identified in Brazil, according to the report.

ABC News’ Christine Theodorou contributed to this report.