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The US Food and Drug Administration has authorized the release of “numerous batches” of Moderna’s updated Covid-19 booster amid reports of supply problems in some areas.

Updated boosters from Pfizer/BioNTech and Moderna were made available early this month, soon after the FDA authorized them. However, some pharmacy chains have reported trouble keeping Moderna’s shots in stock.

“The government supply of the updated Moderna COVID-19 booster is currently limited, causing appointments for the product to vary across the country,” Walgreens said Tuesday. “The updated Pfizer COVID-19 booster remains available.”

Rite Aid said Tuesday, “We have received limited supply of the Moderna COVID-19 booster and expect to receive additional supply in the coming weeks. We are actively working to increase inventory of COVID boosters and have sufficient supply of Pfizer updated boosters in all stores.”

FDA spokesman Michael Felberbaum said Tuesday that when the agency authorized the updated boosters, it was in the middle of an inspection of a facility where Moderna’s booster is made, and thus “FDA did not include this facility as an authorized manufacturing facility for the updated COVID-19 vaccine booster at that time.” Only Moderna batches from another facility were made available at that point.

But on Tuesday, the agency said it had determined that “numerous batches” of the Moderna booster made at the first facility met the criteria for distribution “following a careful review of information provided by Moderna about the manufacture of these batches. The agency has no concerns with the safety, effectiveness, or quality of these batches,” Felberbaum said.

Moderna said it’s still on track to deliver 70 million doses of its updated vaccine by the end of 2022.

“We are working closely with U.S. Government to deliver significant amounts of updated, bivalent booster doses as we continue to see high demand in certain areas of the country,” the company said. “We anticipate that these availability constraints will be resolved in the coming days.”

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Uptake of Covid-19 vaccine boosters had been low long before before the updated boosters were available. To date, less than half of people in the US who received their initial series and are eligible for a booster – and less than a third of the population overall – had gotten their booster shot.

The federal government has not released data on how many people have gotten the new shots, but the authorization of the updated boosters may have spurred more people to roll up their sleeves. Federal health officials have urged people to get an updated Covid-19 booster when they get a flu shot this fall, and the government recently launched an ad campaign targeting people 50 and older.