Headlines: vegetarian diets prevent severe COVID! Study: cross-sectional survey-based investigation of an online sample of healthcare workers shows some interesting associations The headlines are wildly silly 1/npic.twitter.com/MzXS7HUVey
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4/n Anyway, regardless of the study type, it definitely doesn't support the headlines Firstly, there's the population. This study was almost exclusively comprised of middle-aged, white doctorspic.twitter.com/hX9S5uFJh0
5/n We can quibble about generalizability, but as a general rule if ~80% of your population is a single very specific demographic, it's hard to say much about populations based on that sample
6/n Secondly, there's the issue with unmeasured confounding. The study only asked people about a small number of covariates, but there are ~dozens~ of potentially important things that this could've missedpic.twitter.com/8mcUQEyQvQ
7/n Ignoring the obvious things like income, we might be worried that level of exposure to COVID-19 cases could cause people to have a higher risk of COVID-19, but that information wasn't included in the model
8/n There's also some indications in the study itself that the survey data (collected online) might not be very good as well For example, of the 568 people who report having COVID-19, 33% say that they had a NEGATIVE COVID-19 testpic.twitter.com/RvMDkRoQQL
8.5/n This may have been because the definition of a COVID-19 'case' used in the study is very broad. It appears that anyone with any symptoms consistent with COVID-19 would be a case here, regardless of testingpic.twitter.com/kDd0sOTOz3
9/n When the researchers excluded these PCR-negative cases from their model, the statistically significant association between vegetarian diets largely disappeared 

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10/n Anyway, the headlines saying that vegetarian diets reduce the risk of severe COVID-19 are rather ridiculous. This study just can't prove that
11/n A better headline might be "middle-aged white doctors who report eating vegetarian diets at slightly reduced risk of reporting COVID-19 symptoms, study suggests" Not quite as impressive, perhaps, but a bit more accurate
12/n Oh, also, forgot to mention that the headline "73%" reduced risk is a relative risk, if you calculate the absolute risk difference for severe COVID-19 between the vegetarians and everyone else it works out to be about 4% so there's that too
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