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.
(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...