The CDC changed their guidance with PCR testing so a "breakthrough case" (i.e. a vaccinated person testing positive for COVID) is defined as 28 cycles or fewer, but gave no guidance about changing the PCR cycle count for unvaccinated (which remains in the 35-40 range most places).
This means it takes thousands of times less material to make an unvaccinated person a COVID "case" than a vaccinated person.
