(Yes, I'm aware the fraction is better against severe Delta infection. Yes, I agree 3x reduction is infection rate is a good thing.)
https://www.webmd.com/vaccines/covid-19-vaccine/news/2021082...
That's not what "effective" means. It means that if your probability of getting infected without the vaccine is p, your probability of getting infected with the vaccine is .34*p. One way to think about it is that the "ineffectiveness" is the relative risk P[infected|vaccinated]/P[infected|unvaccinated], and effectiveness is 1-ineffectiveness.
A vaccine can be 100% effective even if P[infected] is negligible.
P[infected] is per unit time and does not account for the dynamics of the situation. For example, if the vaccine is not powerful enough to make R<1, then everybody becomes infected eventually regardless of the vaccine.
WITH A 95% CONFIDENCE INTERVAL OF 26% TO 84%
They only statistically modeled how much their unvaccinated population was already recovered from coronavirus based on estimates of community spread, among other issues with that number.
https://www.cdc.gov/mmwr/volumes/70/wr/mm7034e4.htm
And when you have a CI of something like that size, of course you wind up in the middle of it -- the actual value could very well be closer to 80% due to systematic uncertainties in the modeling.
Interesting that moderna continues to do better, and looking forward to seeing the studies VE versus time since vaccination.