How important is it to cite the earliest report of something? As a general rule I try to put in some due diligence to cite the authors who first provided strong evidence of something that I'm referencing, even if I wasn't aware of their work before I started writing. However, with the biblical flood of COVID-19 literature in the past 9 months, it's a hard to pinpoint some of the firsts. You can sort by date on a lot of databases, but a journal's publication date often doesn't reflect when a report was first made available to the public, especially in the era of pre-prints. I've got citations that I know I read in March or April showing up with publication dates of July or August.
Anyways, I'm referencing evidence of COVID-19 (SARS-CoV-2 virus) shedding in stool. At this point there are dozens, if not hundreds of case reports, cohort studies, meta-analyses, and reviews covering this. Does it really matter if I find the first reported case?
TBH, I might be a little bummed if some random review paper were cited for a thing I first proved, but it's also not like this took years (or even weeks) of work to confirm.
For your readers, not important. Very often, later reports or review articles are better than the first report in terms of clarity and completeness. If you did not even need to read that first report when doing your research, then probably your readers do not need it either.
For the academic career game, mildly important. Depending on the field of research, citations may play a role in careers, and citing that first report may provide a (small) boost to its authors.
A pragmatic approach is to not waste too much time chasing citations, and wait for others to recommend citations that you may have omitted. Not everyone has to know the whole literature. And many people are not shy about asking colleagues to cite them (or others). To some extent, you can count on the readers of your article in draft/preprint/submitted form for pointing omissions.