Only a few weeks ago, very few people had ever heard of the condition called cerebral venous sinus thrombosis. After all, it was an extremely rare one.
Then cases of these brain thromboses began to happen after coronavirus vaccinations with AstraZeneca - more than were statistically to be expected.
Several authorities, including the European Medicines Agency (EMA), took a close look at the cases. The result: the benefit of the vaccination clearly outweighed the risk.
"The vaccine saves lives," concluded Peter Arlett, the EMA's chief data analyst.
Low risk, but lost confidence
And yet many were left with a bad feeling about the AstraZeneca vaccine.
"Because of all these media reports and the attention on this topic, people actually overestimate their own risk of contracting thrombosis," virologist Sandra Ciesek from Frankfurt University Hospital said in a recent podcast from broadcaster NDR.
It's quite clear "that the vaccine is far safer than the risk of a Covid-19 infection."
Petra Dickmann, an expert in risk communication, gives another explanation. "People are not rational beings," the doctor explains. A probability refers to an overall population, "but people have to make individual decisions: What do I do for myself?"
In Germany, where more than 4.2 million initial vaccinations with AstraZeneca have taken place, there were 59 cases of cerebral thrombosis by mid-April, including 12 deaths.
If such thromboses are diagnosed and treated early, the chances of a full recovery are relatively good. These blood clots occur on average in about one in 100,000 vaccinated people, according to the analysis of the EMA experts on the AstraZeneca preparation presented on Friday. This translates to a 0.001 per cent risk for the general population.
For comparison: The estimated mortality of people infected with the coronavirus during the first wave was 0.86 percent, according to a Munich study on the proportion of deaths related to all infected persons including the number of unreported cases.
But this figure is also just an average: The proportion of deaths is significantly lower in younger people and significantly higher in older people.
Brain thrombosis due to Covid-19 more likely
Even assuming that infectious mortality has decreased over the past year, experts estimate that the risk posed by Covid-19 to all adults is higher than the risk of a clot after the AstraZeneca vaccine.
Even a 20-year-old woman has a higher risk of severe Covid-19 than of brain thrombosis after the vaccination, says immunologist Carsten Watzl of the Leibniz Institute for Labour Research at TU Dortmund University.
This is confirmed by a study from Oxford University, according to which, the risk of brain thrombosis from Covid-19 is generally many times higher than after vaccination with AstraZeneca.
At the same time, a study from Scotland suggests that the vaccination efficiently prevents severe cases of Covid-19 - even after the first dose. Both studies have not yet been published in scientific journals.
Benefits and risks in different age groups
Researchers from the University of Cambridge have meanwhile used data from British vaccinations to calculate the benefits and risks of inoculations in detail, separating the risks for different age groups and areas with different coronavirus infection rates.
Their results: For 60 to 69-year-olds in a British high-risk area, the risk of having to go to an intensive care unit with Covid-19 within 16 weeks is more than 600 times higher than the risk of cerebral thrombosis after a vaccination with AstraZeneca.
Even in the rarely severely affected age group of 20 to 29-year-olds, the statistical probability of ending up in an intensive care unit with Covid-19 within 16 weeks is twice as high as the risk of a blood clot in the brain after vaccination, with a medium-to-high number of cases for the UK (7-day incidence of 420).
The only exception is the 20-29 age group combined with a low - by UK standards - 7-day incidence of 140, where the Covid-19 risk for the period is a little lower than the vaccine-related risk of thrombosis - but only in the first 16 weeks.
Over time with immunity, the benefit increases, while the blood clot risk is only limited to the first weeks after vaccination.
"Presenting and explaining important facts over and over again makes sense," says communication expert Dickmann. But besides that, trust is also important, he says.
Once trust has been lost, the presentation of facts alone is not enough, says Dickmann. Then only an overarching communication strategy helps rebuild trust.
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