Secretary of Defense Lloyd Austin is expected to announce his recommendation to President Joe Biden that COVID-19 vaccines be made mandatory for troops, officials told ABC News Wednesday evening.

       A senior official said the announcement will come "soon," while a separate U.S. official said an announcement is expected by the end of this week.

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       The president last week directed the Department of Defense to look into how and when vaccines could be mandated for service members. Austin's recommendation in response to that request is expected to be in favor of vaccine requirements, but for Austin to implement such a policy, he'll need a written waiver from Biden.

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       Syringes and vials of the Pfizer-BioNTech Covid-19 vaccine are prepared to be administered in Reno, Nevada, on Dec. 17, 2020.

       Because COVID-19 vaccines are available to the military under the Food and Drug Administration's emergency use authorization (EUA), the shot has so far been strictly voluntary.

       One week ago, Austin told reporters "at least 70% of the force" has received one dose or more, a number comparable to the Centers for Disease Control and Prevention's reporting of 70.2% for all 18-year-olds and up in the U.S. population.

       Pentagon officials have publicly said they would consider requiring COVID-19 vaccinations, as is done with more than a dozen other vaccines, after the FDA fully approves the vaccines.

       "I believe that when it's formally approved, which we expect pretty soon, we probably will go to that, and then that question will kind of be moot," Vice Adm. John Nowell told a sailor in a town hall question-and-answer video posted to Facebook last month.

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       It's reasonable that the FDA will fully approve the Pfizer vaccine by early September, a senior White House official familiar with the FDA approval process told ABC News Tuesday night.

       However, while the two-shot Pfizer vaccine is considered suitable for most troops, the single-dose Johnson & Johnson is preferred in some cases, such as for those who are deploying overseas or aboard ships. A waiver from Biden would mean the DOD wouldn't have to wait for all of the vaccines under EUA to be fully approved before being able to require them, which would afford the Pentagon more options.