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Ken

You're arguing here for using absolute risk, rather than risk ratio, which in many cases is a better guide. It isn't for vaccines because the risk is quite variable, depending on country. Risk of having had covid ranges from less than 1 in 10,000 up to almost 1. For a country where everyone has covid the absolute risk reduction is 60-80% from being vaccinated. Without vaccination everyone would have covid.

Kaiser

Ken: Nothing in this post re-defines VE though. I'm just using the definition as used in the vaccine trials.

You raise two interesting issues about VE definitions.

1. "Without vaccination everyone would have covid"

This would imply we don't need a control/placebo group. But the statement is true only if we let time -> infinity, and not true within any study period.
Also think about VE for deaths. The analogy would be "without vaccination everyone would die from covid"

2. Relative risk ratio and absolute risk (difference) are both useful metrics, depending on the situation. With vaccines, the community has decided on risk ratios. With ratios, we are assuming that if the vaccine cuts infection by 50% on a base rate of 10% in one place, then the vaccine would cut infection rate from a base rate of 1% to 0.5% in another place. Yes, this allows us to apply the result to any regions and it's easy to forget that VE being the same everywhere regardless of base rate is an assumption. I don't think this assumption is obviously true.

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