Test-negative case control studies are abundant in the era of COVID, and are the primary type of observational study used to calculate vaccine efficacy. As reviewed here, this design was first used in in 1980 for the pneumococcal vaccine. Its use rapidly increased in the last decade, and since 2011 it has been the design of 90% of vaccine efficacy studies.
No virus has ever been isolated allegedly, have you looked into that? Here is a good start if you havent.
https://andrewkaufmanmd.com/sovi/