The COVID-19 pandemic

[tomh009]

Do you know why the trials were so different in size? Is there not a standard stage 3 study size?

There isn't really a fixed minimum trial size. The trial size will, though, factor into the confidence level for the results and the regulators are more likely to approve a vaccine that had a large trial population, given similar results, as the confidence levels will be much higher (just as with polling).

10,000 is fairly low as far as stage 3 trials go, some go up to 100,000 people, but the 30-40,000 for Pfizer/BioNTech and Moderna is pretty much mid-range. Here is a chart from NIH that shows a number of vaccine trials with the stage 3 trial size on the x-axis (vaccine cost on the y-axis but that's a different discussion).