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Pfizer is expected to seek US Food and Drug Administration emergency use authorization for a coronavirus vaccine booster shot for people 18 and older, a Biden administration official said Monday. The request could come as soon as this week, although the date could shift.

The expected EUA request was first reported by the Washington Post.

Authorization would bring the country one step closer to President Joe Biden’s mid-August prediction that boosters would be available for all adults in the United States. It also comes as federal health officials have made clear their concern about waning immunity as the nation heads into the winter months.

The FDA has already authorized Covid-19 vaccine boosters for the majority of adults once they’re far enough past their initial doses.

People who got the Pfizer/BioNTech or Moderna Covid-19 vaccines six months ago or longer may get a booster if they are 65 or older; at risk of severe Covid-19 from a breakthrough infection because of a medical condition such as diabetes, kidney disease or pregnancy; or at risk because of living conditions or work. Anyone who got the Johnson & Johnson shot two months ago or longer is eligible two months after getting their first shot. The J&J vaccine is only authorized for people age 18 and older.

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Pfizer said Monday the company has no update about whether it will seek booster authorization for additional groups.

More than 24 million fully vaccinated people, about 12.4% of the total, have already received a booster dose of vaccine.

CNN’s Amanda Sealy contributed to this report.