Johnson & Johnson (JNJ 0.59%) announced late-stage results for its COVID-19 vaccine in late January. The immediate focus was on the vaccine's efficacy, which was markedly lower than efficacy levels reported by Pfizer (PFE -0.80%) and Moderna (MRNA 12.47%) for their COVID-19 vaccines.

In this Motley Fool Live video recorded on Feb. 1, 2021, Motley Fool contributors Keith Speights and Danny Vena discuss one reason why you can't really compare J&J's results with the results reported by Pfizer and Moderna.

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Danny Vena: One of the things that I wanted to bring up and maybe you could give us some color on this. I was watching one of the news reports when it first came out, when they announced that Johnson & Johnson was going to be releasing the data on their vaccine.

Dr. John, who's on one of the morning shows, basically, what he said was, when we're looking at a 60 or 70 percent rate versus a 90 something percent rate, it was an apples-to-oranges comparison because the studies had different endpoints. Can you give that a little color because that's something that I'm not sure everybody would understand?

Keith Speights: Yeah. I would have to pull out, Danny, the specific results from these companies to see how their endpoints differ, but there is one notable difference that I'm very aware of. Johnson & Johnson only looked at moderate to severe cases of COVID-19, whereas the Pfizer and Moderna studies looked at all cases.

You might think is that tipping the scale to help Johnson & Johnson come out with a higher efficacy than they otherwise would have? But that's not necessarily the case, because even though they're not looking at the mild cases, there's no guarantee that that would've really materially impacted their efficacy one way or the other. Because there are a lot more milder cases, but there would've been a lot more milder cases probably in the placebo group as well as on the group that took the vaccine.

The comment that it's hard to compare these is exactly right because they're not apples-to-apples, and I think the biggest issue is the introduction of these new variants into the J&J study.