Last week was a very good one for Moderna's (MRNA 0.69%) COVID-19 vaccine mRNA-1273. An advisory committee to the U.S. Food and Drug Administration (FDA) unanimously voted to recommend Emergency Use Authorization (EUA) of the vaccine. The FDA then quickly followed through with granting EUA to mRNA-1273. However, it wasn't such a great week for Moderna's shares. In this Motley Fool Live video recorded on Dec. 16, 2020, healthcare and cannabis bureau chief Corinne Cardina and writer Keith Speights talk about why Moderna stock fell on good news last week.

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Corinne Cardina: Turning now to this week, of course, Moderna is top of mind. The FDA advisory committee that did recommend Pfizer (PFE 0.29%) and BioNTech's (BNTX 1.64%) vaccine be authorized, they are reconvening tomorrow to discuss Moderna's vaccine candidate. Moderna's stock, it's down almost 7% from yesterday. We'll get into the FDA documents in a moment. But what exactly is going on with the stock?

Keith Speights: I think there probably are two factors at play.

For one thing, Moderna's stock fell yesterday as well, and part of that decline, I think probably stemmed from Dr. Anthony Fauci's comments about just some uncertainties about whether or not we'll need these vaccine shots every year or not. We really don't know at this point, and I think that probably rattled investors a little bit who might have been banking on, "Oh, this is going to be steady recurring revenue from Moderna over time." It will be, but we just don't know how frequently vaccinations will be required. I think that was part of it.

Then the other part of it is something we've talked about already about with Pfizer, what happened last week, that there's a little bit of "buy the rumor, sell the news" involved here. I think some investors have seen Moderna stock just skyrocket this year. And now that they're right at the threshold of the FDA advisory committee meeting and potential emergency use authorization for their vaccine, I think they are saying, "Hey, let's take some of this profits off the table." And we're seeing that happen.

Could there be some other factors at play? Maybe there are some things we don't know about, but I think those are the two biggies right now.