President Biden received backlash on social media Thursday over a tweet that some users saw as a thinly veiled ultimatum when it comes to receiving a COVID-19 jab.

Biden, who addressed the media in the Rose Garden earlier Thursday, announced that the Centers for Disease Control and Prevention updated its mask guidance for fully vaccinated Americans. The guidance allows those vaccinated to go mask-free in outdoor and most indoor settings. The president called it "a great day for America." 

Biden’s official Twitter account later posted, "The rule is now simple: get vaccinated or wear a mask until you do. The choice is yours."

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The tweet was quickly picked up by some influential users. Glenn Greenwald, the journalist, retweeted Biden’s comment and posted, "This is not actually a rule."

The tweet also raised questions about children who don't have the approval to receive the vaccines. Dana Hull, a Bloomberg reporter, tweeted, "Really hard for parents who have kids under 12 to square this."

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The Biden administration has been focused on getting more Americans to sign up for COVID-19 vaccines that it says are safe and highly effective. States have been joining the effort and have been coming up with ways to entice Americans to sign up for a jab. (Ohio just announced a $1 million lottery for those fully vaccinated.)

One of the lingering concerns for Americans with "vaccine hesitancy" is the idea that the drugs were rushed along to meet the urgency, a claim that health officials have denied. 

Some Americans point out that although Pfizer and Moderna have been granted emergency use authorization, these vaccines have not received full approval from the Food and Drug Administration. (Pfizer is seeking full approval and Moderna and Johnson & Johnson are also expected to make the request, ABC News reported.)

The FDA issued a statement this week saying it determined that Pfizer vaccine "has met the statutory criteria to amend" its emergency use authorization and that "the known and potential benefits of this vaccine in individuals 12 years of age and older outweigh the known and potential risks, supporting the vaccine’s use in this population."

The White House did not immediately respond to an after-hours email from Fox News.

U.S. health officials have said it is important to get as many Americans as possible vaccinated so the country gets closer to achieving herd immunity. But since the beginning of the outbreak,  critics say inconsistent messages about mask-wearing and social distancing hurt the effort. There has been concern among some Americans who bristle at the thought of the government limiting their freedoms if they do not receive the vaccine, and this Biden tweet will likely do little to allay those fears.

These health officials have insisted that their recommendations have evolved as they learned more about the new virus. 

Vaccines have been credited with playing a major role in limiting hospitalizations and deaths in the U.S. Virus cases are at their lowest rate since September, deaths are at their lowest point since last April and the test positivity rate is at the lowest point since the pandemic began.

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"Anyone who is fully vaccinated can participate in indoor and outdoor activities — large or small — without wearing a mask or physically distancing," Rochelle Walensky, director of the CDC, said at an earlier White House briefing said. "If you are fully vaccinated, you can start doing the things that you had stopped doing because of the pandemic."