People who were initially immunized with two shots of the Pfizer-BioNTech COVID-19 vaccine should receive a booster shot after five months, rather than six, according to a new recommendation from the the Centers for Disease Control and Prevention.

The move comes after the Food and Drug Administration on Monday authorized the change in the Pfizer booster interval, saying that a third shot after five months may "provide better protection sooner for individuals against the highly transmissible omicron variant."

In a statement, CDC Director Rochelle Walensky said urged eligible Americans to receive a booster as soon as possible.

"As we have done throughout the pandemic, we will continue to update our recommendations to ensure the best possible protection for the American people," Walensky said. "... today's recommendations ensure people are able to get a boost of protection in the face of omicron and increasing cases across the country, and ensure that the most vulnerable children can get an additional dose to optimize protection against COVID-19."

Recommendations for booster shots for those who initially received vaccines made by Moderna or Johnson & Johnson have not changed: Moderna recipients should seek their booster after six months; those who received Johnson & Johnson should get one after two months.

Although the FDA also authorized the use of Pfizer boosters for children ages 12 through 15, the CDC has not yet followed up with a formal recommendation. A committee of advisers to the CDC will meet to vote on that topic Wednesday, after which the agency is expected to act.

The move to shorten the Pfizer booster interval comes as the U.S. reported more than a million new cases of COVID-19 on Monday, an eye-popping new record that far exceeds the worst days of last winter's surge. (Hospitalizations and deaths are rising at much slower rates than cases and have not reached the levels seen last winter.)

That case record is likely to be an undercount, experts say, given the widespread availability of at-home tests.

The CDC now estimates that more than 95% of COVID-19 infections in the U.S. are caused by the omicron variant, which appears to be much more transmissible than previous variants — including among those who are vaccinated or have been previously infected with COVID-19.

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