What’s with the news coming out of Europe on the Oxford/Astrazeneca vaccine and blood clots?

Data and Metrics Vaccines

A: So far there is no evidence that the risk of these events after vaccination is higher than would be expected based on normal rates.

Several European countries including Ireland, Denmark and Norway have temporarily suspended use of the Oxford/Astrazeneca vaccine as a precaution while they review recent reports of blood clots following vaccination.

Meanwhile, the European Medicines Agency (EMA) and the UK’s regulatory body (MHRA) have said that there is no indication that vaccination is linked to thromboembolic events.

The EMA’s safety committee is reviewing the reports but said that the evidence showed that the number of thromboembolic events in vaccinated people was no higher than that seen in the general population.

The World Health Organization (WHO) has also encouraged continued use of the vaccine and stated there is no evidence of a causal relationship. The WHO also emphasizes that getting safety “signals” of this sort is a positive thing, since no adverse reports whatsoever would suggest a lack of review and vigilance.

💡While the reports should be investigated, it’s also important to put them in the broader context before assuming a direct link; halting or slowing the roll-out of vaccines could lead to needless cases of COVID-19 which raise many health risks *including* blood clotting and thrombosis.

➡️It can be understandably scary when something bad happens shortly after a medical procedure like a vaccination. The tricky thing about inferring cause and effect from single events is that STUFF HAPPENS every day. In fact *lots* of things happen after we’re vaccinated. It might rain. You might buy a winning lottery ticket.

⚡Or like one participant in the Moderna Trial, you might get struck by lightning (he/she was OK as far as we know but suffered a heart arrhythmia…not due to the vaccine!).
Then how can we identify true adverse events from a vaccine?

First we test this in the clinical trials, where half the group does NOT get the vaccine but doesn’t know whether they did or not. In the OX/AZ trial, fewer than 1% (168) out of 24,000 participants reported a serious adverse event, and slightly more of these were in the PLACEBO rather than vaccine group. This is very strong evidence that the adverse events were not due to the vaccine.
Once out in the real world, we continue to evaluate vaccine safety. We do this by asking the counterfactual question—how many events would we expect to see over this time even with NO vaccinations? If the adverse events are not above the “base” rate that we would expect over the same period of time, the events are likely coincidence rather than caused by the vaccine.
Serious blood clotting problems including deep vein thrombosis (DVT) and pulmonary embolism (PE) have an estimated annual incidence of 1 per 1000 in the adult population, with the risk even higher after age 40. At this base rate, if five million adults are vaccinated in one week, roughly 95 of those would be expected to have a clotting event even in the absence of the vaccination.
According to the EMA as of March 10th, 30 cases of thromboembolic events had been reported among the five million people given the AstraZeneca vaccine in the European Economic Area, which is LOWER than one would expect.

➡️ TL;DR: Given the MILLIONS of vaccines now being delivered daily, all kinds of good and bad things will happen to some of those people shortly after vaccination that have nothing to do with the vaccine.

💥It can be super scary to hear that someone died or had a blood clot, following vaccination. But if someone is just as likely (or more likely) to experience a health event without vaccination, the timing is almost certainly coincidence—and that’s what scientists and data nerds like us are tracking very closely. We’re watching carefully–there have already been literally hundreds of millions of doses given out, and things are looking extremely good for safety.

Watch this space, we’ll provide any important updates as they emerge!

Those Nerdy Girls

Additional reading:
“Covid-19: European countries suspend use of Oxford-AstraZeneca vaccine after reports of blood clots.”

“Ireland halts use of AstraZeneca vaccine following blood clot reports in Norway”

UK regulatory agency MHRA response

Astrazeneca trial results

Incidence of thrombosis

“The curse of the incidental illness: Seen as side effects to Covid vaccinations, ailments may have little to do with them.”

Moderna trial lightning strike

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