I can say that until 2020, a "case" of anything usually meant you were suffering, in bed ill, had symptoms or were in hosptial as a result of having a "case" of something. Like a bad case of flu, food poisoning etc
There's a bunch of exceptions: all those things that are screened for. Like (to take an example at random) Type 2 diabetes: lots of people discover they're diabetic as a result of a routine blood test.
"Case" in the context of Covid-19 should be defined depending on context. For example on
https://coronavirus.data.gov.uk/details/cases they say what they consider a case to be. And the ONS survey says that it's showing an estimate of the number of people who would test positive using the PCR test in the way that they're using it.
Who gets tested is an important variable. Mostly (for a while now) in the UK that's mostly been people with one or more symptoms. (Not exclusively, but mostly.)
(One factor which may or may not be significant is that the symptoms being used are apparently wrong now: people who're infected with the delta variant, especially those who are vaccinated, aren't likely to have any of the symptoms which you ought to have to get a PCR test. On the other hand, the common symptoms are
really common; testing everyone with a runny nose or who sneezes doesn't seem sensible in a Bayesian sense even if it was practical.)
The PCR tests use such high cycle numbers, that one could basically find anything, any remnant of any past virus. From my understanding anything above 35 cycles (millions of duplicates) isn't worth relying on. It appears that most of the labs in the UK have been testing at 40 cycles (a trillion up cycles). The lower the number of up cycles, the more reliable the test result. So anything below 30 could be considered a reliable result.
That's not actually true, though, according to people who're familiar with PCR. See (for one of
many examples)
https://www.mcgill.ca/oss/article/c...st-reliable-despite-commotion-about-ct-values
Take-home message:
- Some people claim, wrongly, that the PCR test for the coronavirus is useless because the so-called Ct values are too high and the test is picking up things that are not the coronavirus
- The PCR tests for the coronavirus that have been internally validated by public health agencies are actually very reliable
- Scientists cannot declare any result above a certain Ct value to be unreliable because Ct values are somewhat relative and must be interpreted by taking into account a variety of factors
Additionally, in the UK we also do sequencing on a large proportion of positive cases.
It's entirely true that just because someone tests positive doesn't mean they're infectious. Or that they were or ever will be infectious. It's known that someone who's recovered from an infection can continue to test positive for a while (perhaps a long while). People work with imperfect information from imperfect tools, as always.
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