genuinely asking, how do we actually know the delta variant is real, and is the dominant variant causing the current cases?
do people actually get tested for specific variants, or is it just a generic covid test?
That said, if you just want to know what proportion of the population has delta, you don't need to sequence that many cases. The point of sequencing a large fraction of cases is to catch emerging variants, that wouldn't be likely to be caught otherwise. But to know whether Delta is 30%, 50%, or 70% of cases, you just need a good random sample, not a particularly large one, and I imagine you can do that at the labs where the PCR tests are being run.
(at one point, I recall that the UK had a test that matched on three distinct sites on the virus, and they found that one of those sites would turn up negative for a particular variant, so they were able to use that as a proxy for the spread of the variant. I don't remember if that was Alpha or Delta though, and obviously it's no replacement for sequencing)
[0] https://cen.acs.org/analytical-chemistry/sequencing/200000-c...
[1] https://www.gov.uk/government/news/uk-exceeds-600000-covid-1...
US does statistical sampling and also prevents patients from being told about variants, https://news.ycombinator.com/item?id=28419280
https://www.cdc.gov/coronavirus/2019-ncov/variants/variant-s...
https://www.cdc.gov/coronavirus/2019-ncov/variants/cdc-role-...
If the lab finds that 87% of the samples from Adam's College are of the Omega Mu strain, then it's assumed 87% of total cases at Adam's College to be Omega Mu variant.
See page [pdf] 5 on this: https://www.doh.wa.gov/Portals/1/Documents/1600/coronavirus/...
It is updated every Wednesday
I've been watching the delta variant take over the world over the past several months here: https://covariants.org
(dark green is delta)
There is also a way to differentiate on some RT-PCR tests (but not all), depending on how they are made.