Thank you, again, for your response.
The CDC says that the Ct value of 28 or less is used for "sequencing" the virus's genome, not for determining breakthrough cases.
As this article (link below) states: "The Ct value ≤28 is not used to define whether a specimen is positive or negative for COVID. It is only used for determining whether a specimen that tests positive could be submitted for SARS-CoV-2 sequencing. It is not specific to vaccine breakthrough cases."
I don't think it's a huge problem that only symptomatic cases are counted as breakthrough cases since the vaccines (at least for the mRNA ones) also protect against asymptomatic infection (link below). After all, widespread testing to detect asymptomatic cases consumes huge resources, which can be saved for something else that's more important. But, of course, counting only symptomatic cases would underestimate the number of breakthrough cases by a small margin.
https://www.cdc.gov/vaccines/covid-19/health-departments/breakthrough-cases.html
