fragments break apart and create mutant versions of the protein
In the spring of 2021, as rare but fatal blood clots shadowed Europe's vaccination campaigns, scientists at Frankfurt's Goethe University offered a molecular explanation — and perhaps a remedy. Their hypothesis pointed not to the vaccines' purpose, but to their architecture: the adenovirus vector technology shared by AstraZeneca and Johnson & Johnson, which mRNA vaccines do not use. It was a moment science has known before, where a promising tool reveals an unexpected edge, and the question becomes not whether to abandon it, but whether it can be refined.
- Rare but sometimes fatal blood clots were appearing in AstraZeneca and J&J recipients, prompting several European nations to suspend their vaccination campaigns entirely.
- German researchers identified a possible molecular culprit: DNA fragments from the spike protein breaking apart inside cells and producing mutant proteins that trigger abnormal clotting.
- Rather than condemning the vaccine platform, the Frankfurt team argued manufacturers could alter specific genetic sequences to prevent the problematic splicing reactions from occurring.
- The WHO urged countries to continue vaccinating despite the concerns, caught between the documented risk of clots and the broader, ongoing toll of COVID-19.
- Fellow scientists immediately flagged the work as an unverified hypothesis — published on a preprint platform without peer review — leaving its validity an open and urgent question.
In May 2021, researchers at Frankfurt's Goethe University put forward a molecular explanation for a troubling pattern: rare but sometimes deadly blood clots emerging in people vaccinated with AstraZeneca and Johnson & Johnson shots. The answer, they proposed, lay in how these vaccines differ from Pfizer and Moderna at a fundamental level.
Where mRNA vaccines deliver instructions directly, AstraZeneca and J&J use an adenovirus vector — a modified virus that carries spike protein DNA into cells. Lead researcher Rolf Marschalek and his team argued that this process sometimes misfires: fragments of the spike protein's DNA break apart and generate mutant protein versions, which in turn trigger abnormal clotting — a condition they called vaccine-induced COVID-19 mimicry syndrome.
Critically, the team did not call for abandoning the platform. Speaking to the Financial Times, Marschalek said manufacturers could be told precisely how to alter the genetic sequences to prevent the unwanted splicing, offering a path forward rather than a dead end.
The study arrived amid genuine public anxiety. Multiple European governments had already paused AstraZeneca rollouts, while the WHO continued to recommend vaccination, weighing rare clotting cases against the wider pandemic toll. The clots were uncommon — but they had killed people.
Other scientists urged caution. Immunologist Adelaida Sarukhan of Barcelona's Institute for Global Health noted that the findings were plausible but unproven, and the research had been posted to a preprint platform without formal peer review. The hypothesis offered both a mechanism and a potential solution — but whether either would survive scientific scrutiny remained, at that moment, unresolved.
In May 2021, researchers at Frankfurt's Goethe University published a study proposing an explanation for a troubling pattern: rare but serious blood clots appearing in people vaccinated with AstraZeneca and Johnson & Johnson shots, sometimes fatal. The culprit, they argued, lay in how these vaccines work at the molecular level.
Unlike Pfizer and Moderna, which use messenger RNA technology, AstraZeneca and Johnson & Johnson employ what scientists call an adenovirus vector approach. The vaccines use a modified virus as a delivery vehicle to instruct cells to produce the coronavirus spike protein, triggering an immune response. The German team, led by researcher Rolf Marschalek, proposed that something goes wrong in this process. When the spike protein's DNA sequences are read by the body's cellular machinery, fragments sometimes break apart and create mutant versions of the protein. These aberrant proteins, the researchers suggested, trigger abnormal blood clotting—a condition they termed vaccine-induced COVID-19 mimicry syndrome.
The implications were significant enough that Marschalek and his colleagues offered a potential fix. Rather than abandon the vaccine platform entirely, they argued that manufacturers could simply alter the genetic sequences in their formulas to prevent these unwanted DNA splits from occurring. Speaking to the Financial Times, Marschalek stated that his team could tell companies exactly how to mutate these sequences so the spike protein would be encoded in a way that avoided the problematic splicing reactions.
The timing of the study reflected real-world anxiety. Several European nations had already suspended AstraZeneca vaccination campaigns after reports of blood clots in vaccinated individuals. The World Health Organization, however, had recommended that countries continue immunizing despite the concerns and the resistance from some governments. The blood clotting cases were rare, but they were real, and they had killed people.
Yet the study came with a significant caveat. Other scientists in the field immediately cautioned that the German researchers had presented a hypothesis, not proven fact. Adelaida Sarukhan, an immunologist at Barcelona's Institute for Global Health, put it plainly: the finding didn't mean it was wrong, but it needed to be tested and verified by other experts before drawing firm conclusions. The research had been posted on Research Square, a preprint platform, meaning it had not yet undergone the formal peer review process that typically validates scientific claims. The study offered a plausible mechanism and a potential solution, but whether either would hold up under scrutiny remained an open question.
Notable Quotes
With the data we have, we can tell companies how to mutate these sequences, encoding the spike protein in a way that avoids unwanted splicing reactions— Rolf Marschalek, Goethe University
It doesn't mean it isn't true, but they have to prove that it is— Adelaida Sarukhan, immunologist, Institute for Global Health Barcelona
The Hearth Conversation Another angle on the story
Why would the adenovirus vector cause clotting when the mRNA vaccines don't seem to have the same problem?
The German team's theory is that it's about how the genetic instructions get read. With an adenovirus vector, the DNA gets transcribed inside cells, and sometimes those transcription events go wrong—pieces break off and create mutant spike proteins that the immune system treats as foreign threats, triggering clotting.
So they're saying the vaccine itself is working, but a byproduct of how it works is dangerous?
Exactly. It's not that the vaccine fails to work. It's that the delivery mechanism has this flaw built in. The spike protein is supposed to trigger immunity, but if fragments of it mutate, those fragments might activate clotting pathways.
And Marschalek said they could fix it just by changing the genetic code?
That's what he claimed—that you could rewrite the sequences to prevent the unwanted splicing. It sounds simple in theory, but you'd have to test whether the modified version still works as a vaccine and doesn't create new problems.
Why was the scientific community skeptical so quickly?
Because it was a preprint, not peer-reviewed. And because the mechanism, while plausible, hadn't been independently verified. In science, a good hypothesis isn't the same as proof. You need other labs to reproduce the findings.
Did the study change anything about how people were vaccinated?
Not immediately. The WHO kept recommending the vaccines continue. But it added to the uncertainty at a moment when some countries had already paused AstraZeneca out of caution. The study gave a potential explanation for what was happening, which mattered psychologically even if it wasn't yet confirmed.