Depends on what you mean by "effective" (i.e. are you talking about infection, hospitalisation or death?), and relative to what (i.e. ancestral wild type vs Alpha vs Delta).

If you're talking Alpha vs Delta and about hospitalisation/death, then the vaccines are actually marginally more effective against Delta than Alpha according to data from Public Health England (although there's significant overlap in the confidence intervals).

So you don’t get it and pass it on to the unvaccinated in the window that you are infected and your body is fighting it off.