Pfizer (NYSE:PFE) began shipping its COVID-19 vaccine for U.S. use nearly two weeks ago. The label on each vial of BNT162b2 states that there are five doses included. However, some pharmacists discovered that some vials actually have more than five doses. In this Motley Fool Live video recorded on Dec. 18, 2020, healthcare and cannabis bureau editor/analyst Olivia Zitkus and Fool.com writer Keith Speights talk about what's the story behind these "extra" doses of Pfizer's COVID-19 vaccine.

Olivia Zitkus: Let's turn to Pfizer. A lot's happened after the vaccine started its rollout in the U.S. this week. A couple of interesting problems have come to light, and I want to touch on two of them.

You both had a really important one talking about the allergic reaction and all the attention they've gotten from Alaska. Number one, apparently, there might be more doses per vial than originally thought in some of the Pfizer vials that have already been distributed. What does this mean? Is it a happy accident for the hospitals and pharmacies that come across this, or is it a red flag that the company just doesn't know how many doses of the vaccine it's actually distributing?

Keith Speights: [laughs] I think it's happy, but not necessarily an accident, Olivia. It's not uncommon for drug makers to overfill the vials just a little. I think that's exactly what we're seeing here, is that there's some overfilling.

In some cases, pharmacists have discovered that they could get six or seven doses out of a vial that the label says that it only has five doses. Actually, there was even some confusion initially where some pharmacists were throwing away the extra doses they were able to obtain. That's not great, and so the FDA had to come out and say, "No, you can use these doses."

So it's not an accident because, again, drugmakers do intentionally overfill in some cases. There are some reasons for that. Syringes used in different settings sometimes have small effects on the size of the dose. That's one reason for the slight overfilling.

There could even be changes in air pressure at different locations and different altitudes that affect the volume of the vaccine in a vial. There are reasons behind a little bit of overfilling there.

The good news, the happy part of this is that the number of doses that we could have from Pfizer's vaccine is going to be higher than some of the predictions as a result of this. How much? We don't know, but it's certainly good news.

Olivia Zitkus: Sure. So there could be a lot of different explanations for overfill, not all bad.

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