In your example in China they were swab testing for the virus in an attempt to measure/control its spread. That’s different to focusing resources on doing vita

In your example in China they were swab testing for the virus in an attempt to measure/control its spread. That’s different to focusing resources on doing vitamin D testing for a speculative study to determine if there is a link between vitamin D and long Covid. I’m just saying that given limited resources during this global pandemic, the best use of resources at present probably isn’t monitoring 4 million people’s vitamin D levels for this specific study. I’m also saying that not every study has to study every possible factor - in fact this is almost impossible to do! This is a brilliant dataset of millions of people self reporting, and there are things that you can find with self reporting that you can’t find with blood testing (and visa versa). This is why other papers will do a literature review to compile together the results of multiple studies to understand the impact.