The document linked above from the CDC raises the bar by which the RT-PCR test will find a positive result in vaccinated individuals.
A RT-PCR Ct Value much lower than what was used to identify a non vaccinated “case” in 2020 does not offer an apples to apples comparison to 2021. RT-PCR Ct Value was lowered relative to 2020 starting around January 7th, 2021 (from 42 to 35) and now this is evidence of an even higher bar to be considered a positive case (less than or equal to 28) once vaccinated.
Making it more difficult to achieve a positive case means less cases. If you apply the same RT-PCR Ct value to 2020, the case numbers would decrease.
COVID-19 vaccine breakthrough case investigation
Information for public health, clinical, and reference laboratories
“Case definition
A person who has SARS-CoV-2 RNA or antigen detected on respiratory specimen collected ≥14 days after completing the primary series of an FDA-authorized COVID-19 vaccine. “
…
“Clinical specimens for sequencing should have an RT-PCR Ct value ≤28.“
