Health

There's no proof the Oxford vaccine causes blood clots. So why are people worried? | David Spiegelhalter


Stories about people getting blood clots soon after taking the Oxford/AstraZeneca vaccine have become a source of anxiety among European leaders. After a report on a death and three hospitalisations in Norway, which found serious blood clotting in adults who had received the vaccine, Ireland has temporarily suspended the jab. Some anxiety about a new vaccine is understandable, and any suspected reactions should be investigated. But in the current circumstances we need to think slow as well as fast, and resist drawing causal links between events where none may exist.

As Ireland’s deputy chief medical officer, Ronan Glynn, has stressed, there is no proof that this vaccine causes blood clots. It’s a common human tendency to attribute a causal effect between different events, even when there isn’t one present: we wash the car and the next day a bird relieves itself all over the bonnet. Typical. Or, more seriously, someone is diagnosed with autism after receiving the MMR vaccine, so people assume a causal connection – even when there isn’t one. And now, people get blood clots after having a vaccine, leading to concern over whether the vaccine is what caused the blood clots.

Call it luck, chance or fate – it’s difficult to incorporate this into our thinking. So when the European Medicines Agency says there have been 30 “thromboembolic events” after around 5m vaccinations, the crucial question to ask is: how many would be expected anyway, in the normal run of things?

We can try a quick back-of-the-envelope calculation. Deep vein thromboses (DVTs) happen to around one person per 1,000 each year, and probably more in the older population being vaccinated. Working on the basis of these figures, out of 5 million people getting vaccinated, we would expect significantly more than 5,000 DVTs a year, or at least 100 every week. So it is not at all surprising that there have been 30 reports.

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It would be so much easier if we had a group of people exactly like those being vaccinated but who didn’t get jabbed. This would tell us how many serious events we could expect to happen to people that were the result of sheer bad luck. Fortunately, we do have such a group. In the trials that led to the vaccines being approved in the UK, volunteers were randomly allocated to receive either the active vaccine or a dummy injection. Everyone then reported any harms they experienced, but crucially nobody knew if they had received the real stuff or an inert injection. By comparing the numbers of reports from the two groups, we can see how many “reactions” were really owing to the active ingredients, and how many were linked to the vaccination process, or would have happened anyway.

Some kind of adverse events were reported by 38% of those receiving the real vaccine but, rather remarkably, 28% of those who received the dummy also reported a side-effect. This shows that the vaccination process itself causes about two-thirds of all the reported harm. Of more than 24,000 participants, fewer than 1% reported a serious adverse event, and of these 168 people, slightly more had received the dummy than the active vaccine. So there was no evidence of increased risk from taking the AstraZeneca vaccine. The Pfizer trials had similar results, with more mild or moderate adverse events in the vaccine group but almost identical numbers of serious events.

Trials are short and comparatively small, and tend to include healthy people, so we need to collect real-world data as the vaccines are rolled out. In the UK, adverse reactions are reported using the “yellow card” system, which dates back to the days when doctors filled in yellow cards to report side-effects. Up to 28 February, around 54,000 yellow cards have been reported for the Oxford/AstraZeneca vaccine, from around 10 million vaccinations given (the Pfizer vaccine has a slightly lower rate). So for both vaccines, the overall reporting rate is around three to six reports per 1,000 jabs. That means a far greater number of side-effects are reported in the trials than through the yellow card system (of course, one factor in this underreporting may be the yellow card website, which appears designed for medical professionals rather than patients experiencing side-effects).

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The vast majority of the side-effects reported through the yellow card system and in randomised trials are reports of direct reactions to the jab, such as a sore arm, or subsequent general flu-like symptoms of headache, tiredness, fever and so on, which subside in a few days. The most serious problem is anaphylactic reactions, and the advice is not to inject anyone with a previous history of allergic reactions to either a prior dose of the vaccine or its ingredients.

So far, these vaccines have shown themselves to be extraordinarily safe. In fact, it’s perhaps surprising that we haven’t heard more stories of adverse effects. There could well be some extremely rare event that is triggered by Covid-19 vaccines, but there is no sign of this yet. We can just hope that this message gets through to those who are still hesitant because of the misinformation that has been spread about the supposed harm of vaccines, and the unhelpful comments made by some European politicians.

Will we ever be able to resist the urge to find causal relationships between different events? One way of doing this would be promoting the scientific method and ensuring everyone understands this basic principle. Testing a hypothesis helps us see which hunches or assumptions are correct and which aren’t. In this way, randomised trials have proved the effectiveness of some Covid treatments and saved vast numbers of lives, while also showing us that some overblown claims about treatments for Covid-19, such as hydroxychloroquine and convalescent plasma, were incorrect.

But I don’t think we can ever fully rationalise ourselves out of the basic and often creative urge to find patterns even where none exist. Perhaps we can just hope for some basic humility before claiming we know why something has happened.



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