https://www.cdc.gov/vaccines/covid-19/health-departments/breakthrough-cases.html

The good people at the CDC have actually gone ahead and said the quiet part out loud.

As previously announced, CDC is transitioning to reporting only COVID-19 vaccine breakthrough infections that result in hospitalization or death to help maximize the quality of the data collected on cases of greatest clinical and public health importance.

They are no longer counting breakthrough cases unless they rise to the level of hospitalization or death. Put another way: they are only counting breakthrough cases when it is impossible to ignore them.

We know that only 1-5% of positive cases require hospitalization. So there's a possibility that 95-99% of breakthrough cases are not being reported by the CDC.

We also know that the vaccines hypothetically reduce hospitalization. This would further reduce the number of breakthrough cases deemed "relevant" by the CDC.

We also know that a significant number of covid cases present nearly asymptomatically, which means a large number of breakthrough cases will never even be diagnosed, let alone not recorded by the CDC.

Put it all together and there's a possibility that the CDC will only record a fraction of 1% of breakthrough cases.

Why?

to help maximize the quality of the data on cases of greatest clinical and public health importance

This sentence is pure obscurantist nonsense. The CDC is simply pretending that the vast, vast majority of breakthrough cases are medically unimportant while simultaneously pretending that the vast, vast majority of unvaccinated cases are critically important. The end result will be an insanely skewed and dishonest data set that fraudulently indicates enormous efficacy for the vaccines. And because journalists are more likely to launder bad science than criticise it, we will be seeing plenty of articles claiming that the vaccines magically prevent infection/transmission despite this never being the intended purpose of these vaccines.

Edit: imagine what covid-19 case numbers would have looked like if they'd only ever counted cases requiring hospitalization. The information would have been almost meaningless. Yet that's exactly how the CDC is counting breakthrough cases now. In the name of "quality data". Because ignoring data is how you do science in 2021.