The military will “immediately begin” vaccinating service members against COVID-19, Defense Secretary Lloyd Austin said in an Aug. 24 memo.

Austin’s memo comes after the Food and Drug Administration announced its full approval of the two-shot vaccine made by Pfizer and its partner BioNTech.

“To defend this Nation, we need a healthy and ready force,” Austin said in the memo. “After careful consultation with medical experts and military leadership, and with the support of the President, I have determined that mandatory vaccination against coronavirus disease 2019 (COVID-19) is necessary to protect the Force and defend the American people.”

Austin directed the service secretaries to “immediately begin full vaccination of all members of the Armed Forces under DoD authority on active duty or in the Ready Reserve, including the National Guard, who are not fully vaccinated against COVID-19.”

Military leaders have for months encouraged service members to get vaccinated. More than 800,000 service members have not yet been vaccinated, according to Pentagon data, the Associated Press reported.

As of Aug. 18, more than 1 million service members—more than 388,000 of them soldiers—have been fully vaccinated, according to DoD data. An additional 244,000 troops, including 174,400 soldiers, are partially vaccinated. 

Mandatory vaccinations will only use vaccines that have received full licensure from the FDA, the memo states. Service members who are actively participating in COVID-19 clinical trials are exempted from mandatory vaccination until the trial is complete, according to the memo.

Service members can receive an exemption for medical or religious reasons, Pentagon Press Secretary John Kirby said. Those who do not qualify for an exemption but still do not want the vaccine will be given the chance to talk to a physician and his or her chain of command, he said.

“Commanders have a wide range of tools available to them to help their teammates make the right decision for themselves, for their families, for their units,” Kirby said. 

He added that the vaccine mandate is now a lawful order. “We fully anticipate our troops are going to follow lawful orders,” he said.

Service members are considered fully vaccinated two weeks after completing the second dose of a two-dose COVID-19 vaccine or two weeks after receiving a single dose of a one-dose vaccine. Those with previous COVID-19 infection are not considered fully vaccinated.

Austin called on the services to move quickly. “The Secretaries of the Military Departments should impose ambitious timelines for implementation,” he said in the memo. “Our vaccination of the Force will save lives. Thank you for your focus on this critical mission.”

The memo is available here.