[Boudica]

In my locality, those who have COVID symptoms must get tested, Inclusive of being fully vaccinated.

In my country, it is the same, but my point is that I don't believe in the data because COVID can saw as other diseases and winter diseases, but nobody will test it since most of us are vax. If I'm a little weak and constipated, it can be misunderstood.

The recommendation where I am is to take lateral flow tests every 3-4 days regardless of vaccination status. These are provided for free on next day delivery, with about 7 to a pack.
Of course, these aren't fool-proof (although a recent study has found they're actually more effective than people originally thought), but they're better than nothing.

You're right, though. The data is never going to be exactly accurate to reality. They can only operate on data they do have, and without every single person being constantly tested, you're never going to have that perfect picture of the truth.