Health Nerd @GidMK writes: It is not likely that mRNA Covid-19 vaccines may cause blood clots

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4/n As expected, they found a higher rate of reports of CVT for the Astrazeneca vaccine (this is a well-known risk) However, they also found a much smaller but still elevated rate of reports for mRNA vaccines

5/n The authors conclude that this might be a potential safety signal for the mRNA vaccines, because while low this is still statistically higher than we might expect

6/n As such papers go, the study is fine. It’s the sort of boring, everyday epidemiology that I do regularly. There’s a potential risk here that’s important to look into, but not enough information to conclude much at all

7/n The reason you can’t conclude anything from these results is because of the denominator. What the authors are doing here is comparing the PROPORTION OF REPORTS for a specific issue between COVID-19 vaccines and other stuff

8/n But obviously, the proportion of reports that are for CVT for COVID-19 is influenced by many factors, i.e. press coverage of cases of CVT, international monitoring systems etc. It also tells us nothing about the rate of the event, just what % of reports are for CVT

9/n At the very least, you need studies that look at cases of CVT using vaccine doses (not adverse event reports) as the denominator to see if there really is an elevated RISK of the event or if it’s just a reporting quirk

10/n …and luckily, THESE HAVE ALREADY BEEN DONE For example, this large study looking at mRNA vaccines in Singapore. No large increased risk of CVT

13/n I could go on. There are independent published reviews into the question of CVT for mRNA vaccines from a half-dozen regulatory agencies across the world because after the risk was identified in adenovirus vector vaccines it was important to check for mRNA as well

14/n But the point is that we’ve investigated the vague correlation suggested by this paper, and found that, in larger and better-controlled studies, the risk of CVT after mRNA vaccines is not elevated

15/n What is bizarre about this all is that the original paper doesn’t mention any of the research that has already answered the questions they raise. The authors just say this:

16/n In fact, there are at least a dozen such studies that I could find, probably more, which raises the question of whether the authors actually checked before submitting their work

17/n Anyway, bottom line: – this study raises a possible connection btwn CVT and mRNA COVID vaccines – however, it’s not very well-controlled – that risk does not appear to be seen in more rigorous research – therefore, it’s unlikely that mRNA vaccines cause CVT

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