Millions of Americans have already been fully vaccinated against COVID-19. However, there's a real possibility that another booster shot could be needed. In this Motley Fool Live video recorded on April 21, 2021, Motley Fool contributors Keith Speights and Brian Orelli answer a viewer's question about what's going on with these booster vaccines.

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Keith Speights: Elizabeth has a question. She asks, are there any updates on the development of the Pfizer (PFE 0.88%) and/or Moderna (MRNA -0.15%) booster shots that are projected to be possibly needed in the near future?

Brian Orelli: Yeah. I think they're in clinical trials, is that right?

Speights: Yes. They're taking a little bit different approach. Pfizer is in clinical studies with literally just a third booster shot of their existing vaccine, and Moderna though, they're testing of variant specific shot.

Orelli: Right and I think Novavax (NVAX -7.42%) is at least developing a variant specific shot although I haven't seen that they are in clinical trials yet.

Speights: Pfizer had talked about also pursuing a variants specific shot but I haven't seen anything yet anyway that led me to believe that they are in clinical testing with that at this point. That's where we are.

I think Moderna CEO said that the company expects to have a booster shot available by the fall, or certainly before the end of the year, and I think you've said by the fall.

Orelli: Yes. I think the thing here is probably they're just going to look at antibody levels. They're not going to try to measure the efficacy based on placebo-controlled clinical trial where they would look for whether you'd get COVID-19, and so that allows them to your smaller trials and also allows them to get the data a lot quicker than having to actually measure efficacy in the development of COVID the disease.