Military troops must immediately begin to get the Covid-19 vaccine, Defense Secretary Lloyd Austin said in a memo on USWednesday, ordering service leaders to “impose ambitious timelines for implementation.”

More than 800,000 service members have yet to get their shots, according to Pentagon data. And now that the Pfizer vaccine has received full approval from the Food and Drug Administration, the Defense Department is adding it to the list of required shots troops must get as part of their military service.

The memo does not dictate a specific timeline for completing the vaccinations. But it says the military services will have to report regularly on their progress. A senior defense official said that Austin has made it clear to the services that he expects them to move quickly, and that this will be completed in weeks not months.

“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…is necessary to protect the Force and defend the American people.”

Troops will be able to get their Pfizer shots at their bases and from their commands around the world. The Pentagon has said it has enough vaccine supply to meet demand. Individual service members may also go out and get any of the other COVID vaccines on their own.

Fulfilling the vaccine mandate, however, may be a challenge for National Guard forces who are scattered around the country, and gather just once a month for their required drills.

According to the Pentagon, there are more than 1.3 million troops on active duty and close to 800,000 in the Guard and Reserve. And, as of Aug. 18, more than 1 million active-duty, Guard, and Reserve servicemembers were fully vaccinated and nearly 245,000 more had received at least one shot.

Senior military leaders have consistently pressed their forces to get vaccines through a wide range of public pleas, and via social media and other campaigns. But — as is true among the U.S. population — many service members have been reluctant.

Defense officials have said it’s critical for troops to get the vaccine because they live and work closely together and outbreaks could hamper the U.S. military’s ability to defend America.

Military officials have said they don’t have specific numbers on how many Guard troops are not yet vaccinated, and the Pentagon only provides a troop total that lumps active duty, Guard and Reserve into one statistic.

Guard officials have said all along that it is very difficult to assess how many of their citizen-soldiers have gotten a vaccine. And only now will they be able to begin actually tracking the number with more precision as Guard members report to their drill weekends this fall.