Defense Secretary Lloyd Austin is vowing he “won’t let grass grow under our feet” as the department begins to implement the new vaccine and testing directives. But Pentagon officials on Friday were scrambling to figure out how to enact and enforce the changes across the vast military population and determine which National Guard and Reserve troops would be affected by the orders.

The Pentagon now has two separate missions involving President Joe Biden’s announcement Thursday aimed at increasing COVID-19 vaccines in the federal workforce. The Defense Department must develop plans to make the vaccine mandatory for the military, and set up new requirements for federal workers who will have to either attest to a COVID-19 vaccination or face frequent testing and travel restrictions.

Austin said the department will move expeditiously, but added that he can’t predict how long it will take. He said he plans to consult with medical professionals as well as the military service leaders.

Any plan to make the vaccine mandatory will require a waiver signed by Biden, because the Food and Drug Administration has not yet given the vaccine final, formal approval. According to federal law, the requirement to offer individuals a choice of accepting or rejecting use of an emergency use vaccine may only be waived by the president, “only if the president determines in writing that complying with such requirement is not in the interests of national security.”

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Mandating the vaccine prior to FDA approval will likely trigger opposition from vaccine opponents, and drag the military into political debates over what has become a highly divisive issue in America.

Military commanders, however, have also struggled to separate vaccinated recruits from those not vaccinated during early portions of basic training across the services in order to prevent infections. So, for some, a mandate could make training and housing less complicated.

Military service members are already required to get as many as 17 different vaccines, depending on where they are based around the world. Some of the vaccines are specific to certain regions. Military officials have said the pace of vaccines has been growing across the force, with some units seeing nearly 100 percent of their members get shots.

According to the Pentagon, more than 1 million service members are fully vaccinated, and another 233,000 have gotten at least one shot. There are roughly 2 million active duty, Guard and Reserve troops.

A vaccine mandate will also raise questions about whether the military services will discharge troops who refuse the vaccine.

National Guard officials said initial guidance suggests that Guard troops who initially refuse the vaccine once its mandatory will receive counseling from medical personnel. If they still refuse they would be ordered to take it, and failure to follow that order could result in adminitrative or punitive action.

On Friday, Guard officials said leaders were still nailing down legal recommendations on which citizen soldiers would be affected by the new requirements and who would not. Officials said it appears the bulk of the Guard would eventually have to get the vaccine, when it is mandated.

Guard troops on federal active duty would be given the vaccine in their units wherever they are deployed, and others would get it when they report to their monthly drill weekend or annual training. The system, according to Guard officials, would resemble any other vaccine requirement.

Guard members who are on state active duty would not be subject to the requirement initially because they are subject to state laws. But once they return to a monthly drill, the order would apply to them. Guard officials spoke about the new vaccine process on condition of anonymity because procedures are still being finalized.

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While the number of COVID-19 deaths across the military has remained small —- largely attributed to the age and health of the force — cases of the virus have been increasing.

As of this week, there have been more than 208,600 cases of COVID-19 among members of the U.S. military. Of those, more than 1,800 have been hospitalized and 28 have died.

Earlier this year, the number of cases and hospitalizations had been growing by relatively small, consistent amounts, and the number of deaths had stalled at 26 for more than two and a half months. In recent weeks the totals spiked. The number of cases increased by more than 3,000 in the last week alone, and those hospitalized grew by 36. Two Navy sailors also died in the last week.