WASHINGTON — All but eight of the detainees at Guantánamo Bay have received their first dose of the COVID-19 vaccine, the U.S. Southern Command said Tuesday.

Biden administration officials disclosed that the Department of Defense had decided to expand the availability of the vaccine to all 40 detainees starting Monday, the day all people 16 years and older became eligible for the vaccine in the United States.

The Southern Command, which has oversight of the detention center, first sought permission from the Trump administration to vaccinate the detainees in a Dec. 23 memo that described the prison population at Guantánamo as “a high-risk community,” and invoked both “the Geneva Convention and Department of Defense guidance.”

“Thirty-two of the detainees have received the first dose of the COVID-19 vaccine,” the command said in a statement. Military officials declined to say whether the other eight detainees were offered the first shot of the two-shot Moderna COVID-19 vaccine but refused.

The vaccines are not mandatory for the military or civilian Defense Department employees and were offered to the detainees on a voluntary basis. Many of them are approaching their second decade in U.S. detention and have chronic illnesses. The oldest is 73 and has a heart condition, diabetes and other geriatric illnesses.

The Biden administration froze initial plans to offer them the vaccine on Feb. 1 because elected officials and families of the victims of the Sept. 11 attacks accused the Defense Department of putting terrorism suspects ahead of the American people, who were only just beginning to get access to the vaccines in substantial numbers at that point.

Vaccination rates have risen substantially since then across the country, which was nearing President Joe Biden’s goal of administering 200 million doses by his 100th day in office.

Defense officials also decided to go forward with vaccinating the detainees this week in part because of what appears to be a significant rate of refusal to take the vaccines by adults at the base of about 5,500 residents, including at the detention center, which has a staff of 1,500 guards and civilians. As of April 1, according to health officials at the base, 47% of those eligible had not yet taken a single dose.