How vaccination affects the risk of stroke

When the experts at the European Medicines Agency (EMA) gave the go-ahead for the approval of the Omicron-adapted vaccines last Thursday, they must have noted two important publications with relief. Both testify: Corona vaccinations do not increase the risk of suffering a stroke.

Doctors have been investigating this suspicion for a little over a year. Back then, in March 2021, the first cases of sinus vein thrombosis, dangerous vascular occlusion of the large cerebral veins, occurred after an injection with the vector-based vaccines from AstraZeneca and Johnson

And even if there was no such alarm signal from the mRNA vaccines from Biontech and Moderna, it still raised the question: could corona vaccines in general promote the formation of blood clots in the head? In fact, a major UK inquiry from October 2021 initially appeared to back up this concern. Scientists from the University of Oxford found that in the first and third week after the Biontech vaccination, the number of cerebral hemorrhages increased slightly.

But now two studies with a total of more than 800 million vaccinated people give the all-clear. What's more, another study suggests that the vaccines even protect those infected with Covid-19 from strokes. The team led by Maria-Ioanna Stefanou from the University Hospital in Athens evaluated the data from 16 different studies. They counted almost 17,500 strokes after more than 782 million vaccinations.

That would correspond to fewer than five strokes per 100,000 vaccinations, the authors write in the specialist journal Neurology and is no higher than is usually observed in the population - but about 200 times less frequently than after a Covid infection. The experts from the French regulatory and supervisory authority ANSM came to a very similar conclusion in the journal Annals of Internal Medicine. They searched the national system for health data (Système National des Données de Santé) in vaccinated people between the ages of 18 and 75 for connections between the vaccinations and complications such as strokes, pulmonary embolisms or heart attacks.

The result: vaccinations with the vector vaccines from AstraZeneca and Johnson did work

This now raises another question: How can it be that the British study, although it also included a total of 32 million vaccinated people, came to a different conclusion? The answer to this explains a problem with many initial studies on corona and vaccinations: the period and the group of vaccinated could influence the result. The Oxford study, for example, collected its data between December 2020 and May 2021, i.e. at the beginning of the vaccination campaign.

During this time, it was primarily the elderly and those with previous illnesses who were vaccinated, i.e. people who are already at higher risk of developing heart attacks, strokes or other complications. Different points in time, age structures or population groups also made it difficult to compare different studies or to transfer them to German conditions: the Israeli population is younger on average; in large veteran studies from the USA, however, the participants were often older and sicker. However, the more time passes, the more data from vaccinated people flows into the evaluations, and the more reliable the knowledge about the vaccinations becomes.