It’s weird that there is so little mention of natural immunity as a cause of declining measured “effectiveness”. The vaccine effectiveness should trend towards 0 as the unvaccinated population acquires natural immunity. That doesn’t mean the vaccine isn’t working.
Imagine a study where half gets vax and half doesn't. If population is pristine at outset at end of study VE is 1-ratio between people who get COVID in the two arms.
Now, if there is any prior exposure to the disease among participants (this isn't always known, esp. for something like COVID), and if there is any natural immunity that accrues, this will pull that ratio towards 1 (VE=0)
