@biobook any thoughts on this?
While I've said that I know very little about immunology, what I know about molecular biology, PCR, and clinical testing is even less than that. So it's not only that I can't answer the question, but I'm not even sure I understand what the question is.
Are you suggesting that the large decrease in new covid cases in January is due to a change in the way cases are counted? That the earlier case numbers were artificially high, when the tests with high Ct's were counted as covid cases, and when the Ct cutoff was lowered to 35, the case number was more accurate, and naturally lower.
If that's what you're asking, I still don't know the answer, but this NYT article from the summer might be relevant, where Dr. Mina was advocating from using the lower Ct cutoff. But if I understand it, his reasoning was that the goal of testing was to find cases that needed to be treated and/or isolated to prevent spread, and those with a high Ct were not in that category. The high Ct reflects a very low viral load, which could occur either if the person had gotten over covid, and the positive test was measuring non-infectious viral particles, or if the person had recently gotten infected, in which case the viral load would be higher in a few days, so he says they should just re-test.
So, again, if I understand this, the cases that were positive with a high Ct WERE covid cases, in the sense that they reflected an individual who had been infected. In some cases, that individual would retest and become a definite positive. IN some cases the individual had likely had a definite positive before the current high Ct test.
Not sure if anything I just wrote is meaningful, or even accurate. Really, I know nothing about this, and I'd rely on whatever AsherO and PlatinumGuy are saying.
ETA: the NYT article
https://www.nytimes.com/2020/08/29/health/coronavirus-testing.html