This preprint describes a series of nine clinical case histories in which people experienced a combination of blood clots inside blood vessels accompanied by low numbers of circulating platelets – which are small cells which are involved in the blood clotting process. The people described had received a dose of the Oxford-AstraZeneca vaccine between 4 and 16 days before they started to develop symptoms of this illness. In four of these patients, tests were done on their blood which found evidence of antibodies that activate platelets and these same blood samples could activate platelets although the other factors needed to make this happen varied between the samples. The authors conclude that the condition they are observing is “vaccine induced”.

This study appears to have several limitations. It is not clear whether a standardised case definition was used. It is not clear what rate of occurrence of such cases among people not immunised with the Oxford-AstraZeneca vaccine or any other vaccine in these populations over this period of time is expected or observed to be. Although the authors have made efforts to investigate the pathological mechanism behind the illnesses described and whether it was similar or different among the cases, they only did so in four cases. Of importance, with one exception, the paper does not report whether or not any of the patients had evidence of current or previous infection with SARS CoV2 which is itself a risk factor for blood clotting disorders. Two of the patients had other previously diagnosed pre-existing blood coagulation disorders.

Accordingly, while it is important and useful to have information about these cases published to facilitate diagnosis and management by other physicians of patients who may present in a similar way, the authors’ conclusion that the relationship with vaccination is causative seems insecure at this point and further information on incidence and mechanism in this and other populations is therefore urgently needed to resolve this question.

In the meantime, it is clear that the personal risks of death and serious complications, including blood clots, as a consequence of contracting COVID-19, which can be largely and possibly entirely prevented by this vaccine are currently much greater in almost all countries in the world than any possible but as yet unproven risks of blood clots related to receiving it.

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This preprint implies a causal association between vaccination and clotting events, but provides only the weakest of evidence.

With millions of doses of vaccine given, of course some people who were destined to suffer clotting events anyway will get them after vaccination.

The way to tell if there is a causal link is to demonstrate an excess of such events in recently vaccinated patients. This paper is not designed to do this.

As far as I can see, it provides no evidence of a causal link. The conclusion that “The AZD1222 vaccine is associated with development of a prothrombotic disorder” Is completely unsupported by their findings. In my opinion this research is not of high quality and is completely unable to say whether or not the vaccine causes clotting.

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