Pfizer(NYSE:PFE) and Moderna(NASDAQ:MRNA) await critical FDA advisory committee reviews for their respective COVID-19 vaccine candidates over the next two weeks. The prospects that both vaccines win Emergency Use Authorization (EUA) from the FDA following those reviews appear to be pretty good. However, it's not a total slam dunk. In this Motley Fool Live video recorded on Dec. 3, 2020, healthcare and cannabis bureau chief Corinne Cardina and Fool.com writer Keith Speights discuss what could keep Pfizer's and Moderna's COVID vaccines from EUA.

Corinne Cardina: Looking ahead at these advisory committees that are convening, the first one, December 10th, to review Pfizer, and then December 17th, to review Moderna. Are there any potential challenges that might prevent the FDA from greenlighting either of these candidates? What are they looking for in order to authorize them?

Keith Speights: Americans should realize that there's always that chance. There's that possibility that the vaccines won't get a thumbs-up. I don't think that's likely personally, but there is a chance that we need to acknowledge. Probably the biggest potential gotcha that could arise is if the Advisory Committee, and then ultimately the FDA itself, sees the safety issue in the raw data that we just haven't seen so far yet.

Maybe there's something in there that raises a flag that causes them to say, "Okay, timeout, we're going to need some more information. We can't move this quickly." That's a possibility. I don't think it's a probability, but it could happen.

Safety, of course, is going to be the top priority for the FDA and for the Advisory Committee. Efficacy is probably less of a concern based on what we've seen from the companies so far. Efficacy should be easy. That shouldn't be a problem. But there's always that outside chance that there is a safety issue somewhere there.