There are basically 4 effectiveness numbers, based on whether you measure:

* positive PCR tests

* showing symptoms

* hospitalization

* death

I share kube-system's frustration with the media glossing over those details quite often.

Here in Germany, there were numbers thrown around such as the mRNA vaccines being 90%+ effective, and the AstraZeneca being only in 60ies or 70ies (which was likely based on PCR tests). Which made many Germans hesitant to get that vaccine.

Turns out, AstraZeneca prevents hospitalization at a roughly 90% effectiveness rate (recent numbers from England, don#t have link ready right now), which sound much better than 65% number.

A recent study put the Pfizer vaccine at a 97% hospitalization prevention rate, which is much better than the AZ one. How comparable all these studies is a good question.

https://www.cidrap.umn.edu/news-perspective/2021/05/real-wor...

The US vaccines use a different spike protein than the AZ vaccine. It's likely that they are truly more effective.

Absolute risk will ebb and flow with the abundance of the virus in a population, so it wouldn’t be useful as a comparison or as a predictor for the future since every trial will be different.

I skimmed the article you linked to[1], but I still don't understand the advantage of using the absolute risk reduction rather than the relative risk reduction (or the point of reporting both). It seems it's like the same information expressed differently. I'm guessing that there must be an intuitive advantage like measuring the fuel economy of a car in gallons per 100 miles rather than the usual miles per gallon, but I'm not seeing it. Can you explain further?

[1] Quoting the article's explanation of AAR vs RRR: "The vaccine and placebo groups in Figure 1 each have 100 randomly assigned individuals with no history of infection, and an event is defined as the incidence of infection among all individuals during the course of the trial. The percentage of events in the vaccine group is the experimental event rate (EER) or the risk of infection in the vaccine group (1/100 = 1%), and the percentage of events in the placebo group is the control event rate (CER) or the risk of infection in the placebo group (2/100 = 2%). Absolute risk reduction (ARR) is the disease risk difference between the placebo and vaccine groups, i.e., the CER minus the EER (2% − 1% = 1%). The ARR is also known as the vaccine disease preventable incidence (VDPI). Relative risk reduction (RRR) or vaccine efficacy (VE) is the reduced risk from vaccination, the ARR or VDPI, relative to or divided by the risk in unvaccinated individuals, the CER (1%/2% = 50%)".

> I'm guessing that there must be an intuitive advantage like measuring the fuel economy of a car in gallons per 100 miles rather than the usual miles per gallon, but I'm not seeing it. Can you explain further?

Sure, ARR is more interesting for risk/cost benefits analysis. So much so that it is part of the official FDA guidelines expressed in this document ("Communicating risks and benefits: An evidence based user guide" [1]).

"Another statistical choice is between reporting relative or absolute risks. Because there is no way to infer the latter from the former, absolute risks are always more informative. Doubling a risk means very different things if that entails going from 10% to 20% or from 0.001% to 0.002%.Even when they contain the same information, different summaries can highlight different perspectives, hence bias choices."

Besides, if you compare the ranks of the main vaccines in each stat, you can see that it is not the same information expressed differently:

ARR: AZ 1.3%, Moderna 1.2%, J&J 1.2%, 0.84% Pfizer RRR: 95% Pfizer, 94% Moderna, 67% J&J, 67% AZ

1: https://www.fda.gov/media/81597/download

I share your skepticism. Personally, I have talked to people (educated and intelligent people) who refuse to get the vaccine because "it is only ~1% effective". Using ARR seems to cause more confusion and misunderstanding that RRR.

My understanding is that “effectiveness” in these studies means that it is x% effective at preventing covid-19. So comparing these numbers is not the most important thing. Most all of the vaccines on the market are very, very effective at stopping death and hospitalization, which is the most important thing.

> preventing covid-19

That could still mean several different things.

I'm not questioning whether or not any of them are effective enough to be useful. I got the first one I could. I'm questioning whether or not the comparisons that others are making are meaningful.