The Pentagon is requiring members of the U.S. military to get vaccinated against coronavirus.

All military troops will have until Sept. 15 to get vaccinated, according to a memo sent by Defense Secretary Lloyd Austin to all Department of Defense employees. It could be even sooner depending on developments regarding vaccine approval or the spread of COVID-19.

“I will seek the president’s approval to make the vaccines mandatory no later than mid-September, or immediately upon [FDA approval] whichever comes first,” Austin said in the memo, which went out to troops on Monday.

“I will not hesitate to act sooner or recommend a different course to the President if l feel the need to do so,” Austin added. “To defend this Nation, we need a healthy and ready force.”

The Pfizer vaccine is expected to be approved by the Food and Drug Administration in early September. Should that not happen, Austin could seek a presidential waiver to require troops to get the vaccine.

Austin encouraged all employees to get vaccinated right away, regardless of the requirement.

“To defend this Nation, we need a healthy and ready force. I strongly encourage all DoD military and civilian personnel – as well as contractor personnel – to get vaccinated now and for military Service members to not wait for the mandate,” he said.

President Biden endorsed the idea of a vaccine mandate in a statement issued Monday afternoon.

“I strongly support Secretary Austin’s message to the Force today on the Department of Defense’s plan to add the COVID-19 vaccine to the list of required vaccinations for our service members not later than mid-September,” Biden said. “Secretary Austin and I share an unshakable commitment to making sure our troops have every tool they need to do their jobs as safely as possible.”

According to The Associated Press, troops already have to receive as many as 17 vaccines, depending on where they are in the world.

So far, more than 74% of the Navy have received at least one dose of a COVID-19 vaccine. Other branches of the military lag behind, with 65% of active duty Air Force members and 60% of Air Force reserves having had at least one shot. In the Army, that number is closer to 50%, the AP reported.