What happened

Shares of Moderna (MRNA 0.88%) were slipping 2.9% as of 12:13 p.m. EDT on Wednesday. The decline came after the Biden administration announced plans to offer booster shots of Moderna's COVID-19 vaccine (and the Pfizer-BioNTech vaccine) beginning Sept. 20.

So what

Why did the vaccine stock slide on seemingly good news? For one thing, the availability of the third booster doses announced by the Biden administration really won't impact Moderna's sales. The U.S. government has already purchased enough doses to give all Americans who have been fully vaccinated with the company's vaccine a third shot. 

Gloved hands holding a syringe with needle near a person's arm.

Image source: Getty Images.

Also, the amount of today's move isn't unusual for Moderna even when there aren't any developments. The stock is exceptionally volatile, especially with ongoing concerns about its lofty valuation.

However, the White House decision to move forward with booster doses could potentially help Moderna. If other countries that haven't bought as many doses of Moderna's vaccine follow the lead of the U.S., it could boost the company's sales -- especially in the first half of 2022.

Now what

The U.S. Food and Drug Administration must first complete its review of the safety and efficacy of a third dose of the messenger RNA vaccines. The Department of Health and Human Services is making preparations for the administration of booster shots assuming the FDA will authorize the third doses within the next few weeks.