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Risk of thrombocytopenia and thromboembolism after covid-19 vaccination and SARS-CoV-2 positive testing: self-controlled case series study

BMJ 2021; 374 doi: https://doi.org/10.1136/bmj.n1931 (Published 27 August 2021) Cite this as: BMJ 2021;374:n1931

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Strengthening international surveillance of vaccine safety

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Risk of thrombocytopenia and thromboembolism after covid-19 vaccination and SARS-CoV-2 positive testing: self-controlled case series study

Dear Editor,
As far as I can see the whole cohort of participants were vaccinated. I did ask for clarification from the authors.
In other words it didn't compare vaccinated with unvaccinated.
It compared the risk of disease in the vaccinated with the risk of disease in the vaccinated who also tested positive for Covid19.
So absolutely no conclusions can be drawn on the safety of the vaccines apart from the fact that the incidence of disease was greater than the background incidence of the disease ie among unvaccinated.
Perhaps the authors can clarify if it did compare with unvaccinated, if so how many were in this category as I can see no reference to unvaccinated apart from the possible incidence of disease prior to vaccination.
Ruth Sharratt

Competing interests: No competing interests

03 February 2022
Ruth Sharratt
Associate lecturer
Bangor, Wales