Sen. Ron Johnson, R-Wisc., sent a letter to President Biden and Secretary of Defense Lloyd Austin questioning the Department of Defense's vaccine mandate after receiving reports that troops were receiving shots not licensed by the Food and Drug Administration.

Johnson said that "multiple sources" have told his office that "the Department of Defense’s (DoD) mandatory COVID-19 vaccinations may not be in accordance with Secretary of Defense Austin’s August 24, 2021 memorandum" because they are yet to receive "full licensure" from the FDA.

Austin announced the military vaccine mandate in August, giving active duty troops until Dec. 15 and Reserve and National Guard service members until June 30 to be fully vaccinated with an FDA-approved vaccine. 

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Johnson noted that currently the Pfizer-BioNTech vaccine is the only one with non-emergency FDA approval, yet multiple reports indicate that there currently is not "sufficient approved vaccine available for distribution" to all of those eligible for the shot.

The Wisconsin Republican questioned how the DoD could carry out its mandatory vaccination program given the circumstances, while requesting information on how many service members have been given non-FDA-approved vaccines by the DoD.

Johnson also requested the DoD provide to him all documents and guidelines regarding the vaccine mandate.

"The Senator’s concerns are twofold: that service members are mandated to receive the COVID-19 vaccine and regardless, service members may not be receiving the fully approved vaccine pursuant to the mandate," a spokesperson for Johnson told Fox News.

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The spokesperson said that vaccine mandates for service members and other professionals risk exacerbating "severe worker shortages," most notably in health care settings.

"Vaccine mandates will increase these shortages and degrade our health care system," the spokesperson said. "Reports on how mandates could weaken military readiness should concern every American, especially our commander-in-chief."