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Ongoing uncertainty around Oxford-AstraZenca COVID-19 vaccine and clotting

This article was published on
April 6, 2021

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Several media reports mention comments made by the European Medicines Agency’s head of vaccine strategy, Marco Cavaleri, who told an Italian newspaper: “In my opinion we can now say it, it is clear that there is an association with the vaccine. However, we still do not know what causes this reaction.”

Several media reports mention comments made by the European Medicines Agency’s head of vaccine strategy, Marco Cavaleri, who told an Italian newspaper: “In my opinion we can now say it, it is clear that there is an association with the vaccine. However, we still do not know what causes this reaction.”

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Expert Comments: 

Dr Peter English

There are very many rare conditions. If you compare two groups of people, some of these conditions will, simply by chance, be more common in one group than in another; but such small numbers make it hard to be clear whether it is due to chance or to a difference between the groups. So it is harder still to know whether the putative excess of cases in people who have been vaccinated is due to a causal link, or merely to chance.

Almost invariably, when a new vaccine is introduced, a rare and poorly understood condition is claimed to have been caused by it. This suspicion is amplified by a sort of positive feedback loop.

In this case, central venous sinus thrombosis (CVST) is hard to diagnose, so it is likely that many cases will normally be missed or not reported.1 Given the publicity about the possible association with vaccination, doctors may be more likely to consider the diagnosis of central venous sinus thrombosis in patients known to have recently received a Covid-19 vaccine, so that cases that may otherwise have been missed or not reported are more likely to be reported; and they may also be more likely to ask about vaccination in people in whom the diagnosis is made. If either of these happen it can lead to bias, with the diagnosis more likely to be reported in people who have been vaccinated than in people who have not been vaccinated – whether or not the vaccine is the cause.

To be clear whether a condition (such as CVST) is caused by a particular exposure (such as a vaccine), you have to compare the number of cases in the exposed group with the number you would expect in the same people if they had not been exposed: “O/E” (O for observed in exposed group; E for expected).

The background incidence of CVST – as with many uncommon and hard-to-diagnose conditions – is unclear, so even if we are more confident about the incidence in people who have been vaccinated, it will be difficult to calculate whether we are seeing more cases in vaccinated individuals than we would expect.

When all the data are in it is possible we will discover that the vaccine does not cause even a small increase in CVST cases, and it also remains possible that we will find there is a causal link; but we must put this in context. For those currently being offered the jab, the risk of clotting disorders caused by COVID is many times higher than any possible risks from the vaccine.

It is right that we examine all the data to establish whether there is a link between the vaccine and CVST, but the bottom line remains that it is much safer to be vaccinated than to take the risk of Covid-19 disease.”

1 Alvis-Miranda HR, Milena Castellar-Leones S, Alcala-Cerra G, Rafael Moscote-Salazar L. Cerebral sinus venous thrombosis. J Neurosci Rural Pract 2013;4(4):427-438  PMID: 24347950. (https://pubmed.ncbi.nlm.nih.gov/24347950 or https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3858762/).

Prof Adam Finn

There is a lot that remains unclear about the cases of thrombosis and thrombocytopenia being intensively studied by regulators and widely reported in the media. We need to know more about the people affected and we need to understand exactly how the illnesses came about, while many other questions remain unanswered at this time.

However, there are some things that are very clear. The first is that these cases are very rare indeed. The second is that the vaccines that are available and in use in the UK prevent COVID very effectively. The risks of COVID-19 are real, especially for the middle-aged and elderly people alongside those with medical conditions and occupations in health and social care who are currently being offered first and second vaccine doses.

Although numbers of cases are currently falling, making it less likely that people will get infected in the coming days and weeks, as the lockdown is relaxed we can expect transmission rates to go back up again. The risks of death and serious illness from COVID-19 are therefore much greater for all these people than any possible risks due to vaccination. In short, if you are currently being offered a dose of Oxford-AstraZeneca vaccine, your chances of remaining alive and well will go up if you take the vaccine and will go down if you don’t.

We should learn a lot more about all this in the near future as information comes in from the many countries around the world now giving this vaccine to very large numbers of people. While this happens, we need to make our policy decisions carefully to ensure we maximise public health benefits and protect the public as effectively and as efficiently as possible.

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