WASHINGTON, United States -- U.S. Defense Secretary Lloyd Austin has ordered mandatory vaccination for all active-duty and reserve members of the military, but has not given an exact deadline for the implementation, according to a Pentagon memo dated Tuesday.

In the memo, Austin directed the secretaries of the military departments to "immediately" begin full vaccination of all members of the armed forces on active duty or in the reserve, including the National Guard, who are not fully vaccinated against COVID-19.

Only vaccines that are fully authorized by the Food and Drug Administration (FDA) will be used in the mandatory inoculation campaign, according to Austin. For now, only the Pfizer-BioNTech vaccine has met that standard, after they gained full approval from the FDA on Monday.

Troops who voluntarily received a two-shot regimen or a single-dose vaccine are considered fully vaccinated, Austin said.

The Pentagon chief didn't give an exact deadline for service members to complete their vaccination, requiring instead that service branch secretaries "impose ambitious timelines for implementation" and "report regularly on vaccination completion." According to the Pentagon, there are more than 1.3 million active-duty troops and close to 800,000 in the Guard and Reserve. As of Wednesday, nearly 1.1 million active-duty, Guard and Reserve service members are fully vaccinated, and more than 247,000 more are partially vaccinated, Defense Department's data showed.

Pentagon spokesman John Kirby told a press conference Wednesday that 68 percent of all active-duty troops were fully vaccinated, and "just over 76 percent have at least one dose".

However, vaccination rates vary widely among different branches of service, according to Kirby, with the Navy having the highest full vaccination rate, at 73 percent, and the Army sitting on the lowest, at only 40 percent.