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.

U.S. President Joe Biden's nominee to become surgeon general, Dr. Vivek Murthy, told The Washington Post Live that he thinks the United States has a "good shot" to get all kids in school by the fall.

Murthy, a physician who served as U.S. surgeon general from 2014 to 2017, said the nation must takes these steps: vaccinate staff; ensure schools have the resources to take precautionary measures including PPE and tests; and lower "background rates of infection," which he defines as rates of transmission in the community.

"When those rates are really high, it makes it harder to prevent infection from getting into the school," Murthy said. "So we've got to get that background infection rate down, and we've got to vaccinate teachers and other educators and staff in schools. As well as, when we do have the opportunity to vaccinate kids when the trial data comes through and shows that it is safe and effective, that we've got to start vaccinating children, too."

ABC News’ Anne Flaherty contributed to this report.