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Comment by kragen | Hacker News Reader

> Blood testing 4m people during a pandemic is impractical

On at least two occasions China has swab-tested 10 million people in a single city within a week, when covid re-occurred in a city where it had been thought to be extirpated. I don't think blood testing is twice as difficult as swab testing. So, I don't think it's impractical per se. It's just impractical in the UK.

You know what's impractical? 2.7 million people dying and an unknown fraction of the survivors suffering disabilities and injuries such as diabetes, heart attacks, and cerebrovascular attacks.

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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.

That's a fair point: in China they weren't screening for unknown risk factors like hypovitaminosis D either. And if hypovitaminosis D is a large risk factor (it is, though the causal relationship is unclear—it may just be a comorbidity stemming from some other underlying cause that increases covid risk) studies with much less power should be able to find it (and they did).

Agreed, there are also other limitations with self reporting like you can’t assess the genetic impact and the data will be messier as people misinterpret questions.

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