The CDC is monitoring COVID-19 “vaccine breakthrough” cases at the moment. This means that those who are fully vaccinated with the COVID-19 vaccine can still become infected. According to the CDC, “a small percentage of people who are fully vaccinated against COVID-19 will get sick and some may be hospitalized or die from COVID-19.”
Throughout this pandemic, the tests used to identify “positive” COVID-19 cases has been the reverse transcriptase-polymerase chain reaction (RT-PCR) test, which can detect the virus in nasal swabs (RT-PCR). The PCR test is not actually designed to identify active infectious disease, instead, it identifies genetic material, be it partial, alive, or even dead. PCR amplifies this material in samples to find traces of COVID-19.
The CDC is requiring that clinical specimens for sequencing should have an RT-PCR Ct value ≤28 when conducting tests for vaccinated individuals. “Ct” refers to cycle threshold.
According to Public Health Ontario,
The cycle threshold (Ct) value is the actual number of cycles it takes for the PCR test to detect the virus. It indicates an estimate of how much virus was likely in the sample to start with – not the actual amount. If the virus is found in a low number of cycles (Ct value under 30), it means that the virus was easier to find in sample and that the sample started out with a large amount of the virus. Think about it like the zoom button on your computer, if you only have to zoom in a little (zoom at 110%), it means that item was big to start with. If you have to zoom a lot (zoom at 180%), it means that the item was small to start with.