Sebastian Rushworth 18 March, 2021
An article was recently published in the British Medical Journal that reported on a matched cohort study which compared the risk of dying for those infected with the new British variant (a.k.a. B.1.1.7) and those infected with the older covid variants.
A matched cohort study is a type of observational study where you take a group of people with some condition and then try to find a similar group without the condition to match against. Then you follow the two cohorts over time and see if they differ in some meaningful outcome (like death). Since it is an observational study, it can only show correlation. It can’t prove the existence of a cause and effect relationship, but that doesn’t stop many people acting like it does.
The article has resulted in fear-mongering headlines in news media around the world. Just to take the first example I could find, Al-Jazeera published an article with the headline: “UK variant up to 100% deadlier more deadly, study finds”.
Those darn studies, they’re always finding things. It’s like a never-ending game of whack-a-mole. You knock one down here, and another one pops up over there. Anyway, let’s look in to the study some detail, and see if the claim is true.
There were two criteria that had to be fulfilled for a person to be included in the study. They had to have a PCR-test positive for covid at some point between the beginning of October 2020 and the end of January 2021. And they had to be over 30 years old. The authors don’t provide any reason for the second criterion. The only reason I can see for removing people under the age of 30 is that they pretty much never die when they get covid, and including them would therefore have resulted in less impressive mortality numbers, which would have made it a little bit harder to use the results as part of public fear mongering campaigns.
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