After the U.S. Food and Drug Administration gave full approval to the Pfizer-BioNTech vaccine in August, Defense Secretary Lloyd Austin directed all service branches to move forward with plans to vaccinate their members.
The Army is the latest military branch to respond to Pentagon and White House orders that service members and other federal employees get vaccinated.
Soldiers may request religious, administrative or medical exemptions, although those that have previously been infected with COVID-19 are not exempt.
Soldiers that refuse vaccination could face a reprimand that the Army described as potentially career-ending.
The Army's directive gives Reserve and National Guard units until June 30, 2022, to be fully vaccinated.
Soldiers are considered fully vaccinated two weeks after the completion of a two-dose vaccine, such as those from Pfizer-BionTech or Moderna, or Johnson & Johnson's one-dose vaccine.
"This is quite literally a matter of life and death for our Soldiers, their families and the communities in which we live," Lt. Gen. R. Scott Dingle, the U.S. Army Surgeon General, said in a statement.
"Case counts and deaths continue to be concerning as the Delta variant spreads, which makes protecting the force through mandatory vaccination a health and readiness priority for the total Army," Dingle said.
The Navy and Marine Corps both issued directives on Sept. 1 giving all active duty members 90 days to become fully vaccinated.
The Army has the highest number of service members who are fully vaccinated, at 414,780, according to federal data.
Both the Navy and Air Force have over 300,000 and the Marine Corps 119,014. In total, 1.14 million service members are fully vaccinated.
Service members have been hesitant to get the jab, with a survey conducted earlier this year by military advocacy group Blue Star Families finding that 53% of military families did not plan to get the COVID-19 vaccine.
The Pentagon is preparing to overcome vaccine hesitancy among service members by making sure they are properly counseled about the risks to themselves and their units, Task and Purpose reported.
Those that claim a religious exemption should expect scrutiny. But some are expected to continue refusing the vaccine, even being court-martialed, as some have previously with the inoculation against anthrax.