The COVID-19 pandemic

[danbrotherston]

Is there a reason to use a smaller trial though? I wouldn't think cost would be that big a factor, and recruiting participants should be easy right now.

One factor might be the trial location -- if AstraZeneca did their trial in the UK, and Pfizer/Moderna did it in the US, they would only have one fifth of the population to draw from.

That's pure speculation, though. Normally you would use a smaller trial size to speed up the process and reduce the costs, but, as you say, that shouldn't be the major factor here.

Ironically, the US studies probably have better data on efficacy (but not safety) as a result of the US's record breaking COVID rates.