
To determine the cause or causation, one requires an isolated test (virus) sample. As a scientist, one does not have to, or need to, isolate (isolating) it, but one has to have isolated virus samples to work with. Note that using the words isolating or isolation versus “isolated” – a big difference.
For example, suppose one likes to study sugar for its effects. In that case, one needs isolated, pure, and characterized sugar, independent of its source or process of isolating, whether from sugar cane or sugar beets.
This isolated and pure sample is administered (dosed) in volunteers to see the outcome. Usually, such studies use samples with and without (placebo) test compounds. People often call these clinical trials/studies if such studies are done in humans, otherwise animals, or in vivo.
Medical experts consider such studies to be scientific or science-based. However, in a true sense, these are not scientific studies but surveys, observational, or association studies.
Actual studies are usually conducted at animal, tissue, cellular, and molecular levels to control the variability. More importantly, scientific studies conducted at the molecular level fall under chemical science (science/chemistry), which would require scientifically validated tests for the measurable or quantifiable cause and the response or their validated markers. These are considered mechanistic studies, which can be regarded as causation studies.