“That’s 53 of the control subjects (those receiving a placebo), right?”
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Yes, as you know, of Moderna’s trial participants, half received the new vaccine and half received placebos. Moderna and the FDA will look at the 53 (or more) who develop symptomatic COVID-19 to see how many received the placebo and how many received the actual vaccine. That assessment will determine the efficacy of the vaccine. I think the FDA is looking for vaccines to prevent at least 50% of what would otherwise be an individual COVID-19 case. So, yes, of the 53 symptomatic cases the great bulk should be from the 15,000 participants who received the placebo.
Noted - thanks for the added info. I was not sure what efficacy was actually being looked for, but, that all makes sense.